Possibly, but I think there’s a fine line. He could rant all he wants about whether he just watched on Fox News or saw in a public hearing without causing issues.

Maybe, but I don’t think so. Your read is a valid read of the court opinions, given that they spend a lot of time looking at things like Trump’s use of Twitter for official functions such as hirings and firings and statements to foreign leaders, White House staff’s use of the Twitter account, and plain statements that Trump’s statements on his personal account are official presidential statements. All that stuff is certainly important. But I think even the lower threshold of simply discussing government business would still be enough to classify his account as a limited public forum. From the opinion (pages 4-7):

President Trump established his account, with the handle @realDonaldTrump, (the “Account”) in March 2009. No one disputes that before he became President the Account was a purely private one or that once he leaves office the Account will presumably revert to its private status. This litigation concerns what the Account is now. Since his inauguration in January 2017, he has used the Account, according to the parties, “as a channel for communicating and interacting with the public about his administration.” App’x at 54. The President’s tweets from the Account can be viewed by any member of the public without being signed into a Twitter account. However, if a user has been blocked from the Account, they cannot view the Account’s tweets when logged in to their account. At the time of the parties’ stipulation, the Account had more than 50 million followers. The President’s tweets produce an extraordinarily high level of public engagement, typically generating thousands of replies, some of which, in turn, generate hundreds of thousands of additional replies. The President has not generally sought to limit who can follow the Account, nor has he sought to limit the kind of speech that users can post in reply to his tweets.

The public presentation of the Account and the webpage associated with it bear all the trappings of an official, state run account. The page is registered to Donald J. Trump “45th President of the United States of America, Washington 14 D.C.” Id. at 54-55. The header photographs of the Account show the President engaged in the performance of his official duties such as signing executive orders, delivering remarks at the White House, and meeting with the Pope, heads of state, and other foreign dignitaries.

Following that, the court does go into several paragraphs of detail regarding White House staff and the National Archives stating outright that Trump conducts official government business on his Twitter account. That certainly helps Knight’s case, but it’s not necessary to it; Trump discussing government business on the Twitter account in and of itself would have been enough to qualify it as a limited public forum.

Here’s more, from later in the opinion (page 18, following further discussion of the points you mention about the government repeatedly acknowledging the account as official):

Second, since becoming President he has used the Account on almost a daily basis “as a channel for communicating and interacting with the public about his administration.” Id. at 54. The President utilizes White House staff to post tweets and to maintain the Account. He uses the Account to announce “matters related to official government business,” including high-level White House and cabinet level staff changes as well as changes to major national policies. Id. at 56. He uses the Account to engage with foreign leaders and to announce foreign policy decisions and initiatives. Finally, he uses the “like,” “retweet,” “reply,” and other functions of the Account to understand and to evaluate the public’s reaction to what he says and does. In sum, since he took office, the President has consistently used the Account as an important tool of governance and executive outreach. For these reasons, we conclude that the factors pointing to the public, non-private nature of the Account and its interactive features are overwhelming.

There’s more in the lower court ruling. Pages 53-55 (note that page 54 is cited in the above quotes of the appellate opinion):

In assessing whether speech constitutes government speech as opposed to private speech, the Supreme Court has considered at least three factors: whether government has historically used the speech in question “to convey state messages,” whether that speech is “often closely identified in the public mind” with the government, and the extent to which government “maintain[s] direct control over the messages conveyed,” with Walker’s application of these factors “likely mark[ing] the outer bounds of the government-speech doctrine.” Matal v. Tam, 137 S. Ct. 1744, 1760 (2017) (quoting Walker, 135 S. Ct. at 2246-49); see also Wandering Dago, 879 F.3d at 34 (distilling the same three factors from Walker).

Based on the government speech doctrine, we reject out of hand any contention that the content of the President’s tweets are susceptible to forum analysis. It is not so susceptible because the content is government speech: the record establishes that the President, sometimes “[w]ith the assistance of Mr. Scavino,” uses the content of his tweets “to announce, describe, and defend his policies; to promote his Administration’s legislative agenda; to announce official decisions; to engage with foreign political leaders; to publicize state visits; to challenge media organizations whose coverage of his Administration he believes to be unfair; and for other statements, including on occasion statements unrelated to official government business.” Stip. ¶ 38. Indeed, the content of the tweets sent by @realDonaldTrump are solely the speech of the President or of other government officials. Stip. ¶ 39. For the same reason, the account’s timeline, which “displays all tweets generated by the [account]” is not susceptible to forum analysis: the timeline merely aggregates the content of all of the account’s tweets, Stip. ¶ 15, all of which is government speech.

Note that other officials’ use of the account, his use of the account to engage with foreign leaders, and his official announcements on the account are only some of the factors used to determine that @realDonaldTrump is a public forum, and that other factors include defending his policies, promoting his legislative agenda, and criticizing his coverage by the press.

So I don’t agree with your conclusion that he would have been fine if he’d stuck to ranting about what he just saw on Fox News or in a public hearing. If he were ranting about, for example, the Mueller Investigation, the wall, his tax and healthcare policy — all of those things are directly related to his position as President of the United States. Do you really think that if he’d posted about those topics but the White House had denied that these were official statements, the courts would have ruled differently?

If he had a Twitter account where all he posted was cat videos, then sure, he could block whoever he wants. But he’s talking about government business in his capacity as an elected official.