No, Twitter still isn’t subject to the First Amendment — even if a judge said Trump’s account is

[Analysis] The ruling that President Trump violated the constitutional rights of Americans when he blocked some of his Twitter followers after they criticized him politically raises many more questions about the extent of those First Amendment obligations. President Trump cannot legally block his Twitter followers for political reasons, the judge ruled, because that would amount to “viewpoint discrimination” by a government official in a public forum. 

The ruling pointedly does not rule on Twitter as a whole, nor even the entirety of Trump's Twitter account. It only looks at portions of his account: “The content of the tweets sent, the timeline comprised of those tweets, the comment threads initiated by each of those tweets, and the 'interactive space' associated with each tweet in which other users may directly interact with the content of the tweets by, for example, replying to, retweeting, or liking the tweet.”

The court did not find that Twitter is beholden to the First Amendment. It affirmed the principle that the Constitution applies only to the government and not private individuals. So under Buchwald's ruling, Twitter remains free to block users from its platform, on its own terms, without running afoul of the First Amendment.