ah but the present application of the first amendment is Trump cannot deny people that right by blocking them but apparently Twitter can deny that ability which means for one person the first amendment applies and must be respected on the platform but also it's being argued that it's a private company and private space where those laws don't apply. Doesn't seem consistent there.
Twitter is a private platform. It can accept or deny anyone according to its rights as owner. If it bans anyone, it does not infringe their right to free speech, because there's the whole of the rest of the internet for them to make themselves heard.
However, public servants are required to have a degree of openness to the public about their jobs, because the USA is a not a dictatorship. If a public official uses an open platform to explain their professional activities, they should not unfairly restrict public access. Or to put it another way, the president of the USA is answerable to and has a duty to serve the entire US public. He is not allowed to selectively silence citizens who happen to disagree with him in policy. If Trump had a Twitter account which purely covered his personal life, he (probably) would be able to block users, because it is outside the boundaries of public policy requiring openness.
So you can walk right into the Whitehouse and yell and the president directly?
No, indeed you can't... but no-one can.
So the president is therefore not unfairly disadvantaging some citizens in terms of access to public discussion.