The US government (of which the President heads one branch) is bound by freedom of speech, not private individuals or corporations. Or to let XKCD say it: https://xkcd.com/1357/
If it was a President whom the moderators at Twitter supported and they willingly blocked all of this President's critics on his behalf, would that make it legal?
From the decision: "The First Amendment does not permit a public official who utilizes a social media account for all manner of official purposes to exclude persons from an otherwise-open online dialogue because they expressed views with which the official disagrees...."
I believe the problem is that Trump uses the account for official purposes. I assume that any account that carries out official government business falls under this decision.
> If it was a President whom the moderators at Twitter supported and they willingly blocked all of this President's critics on his behalf, would that make it legal?
If it were on his behalf, I assume the court would consider this equivalent to the President himself blocking them. If Twitter themselves blocked people independently and without colluding with the President, I have no idea. I assume private businesses can continue to block/ban whomever they see fit, but if a public official were to only use platforms that automatically blocked dissenting views, I don't know what the court would say. It's an interesting question.
When they are working with the government, it is not so clear cut. Twitter hosting Trump's tweets is a business relationship between the government and a private business. Otherwise you have created a loophole the size of Alaska that allows government to violate constitutional rights by outsourcing the actual violation.