The crucial part is who is doing the blocking. Twitter as a private company can block, Trump as a member of the government on an official government channel cannot. Companies do not have a constitutional obligation in the first the government does.
Oh there's definitely some path of legal wrangling that may end up compelling Twitter to do something here. In the end whether that's true or not will simply come down to a Judge and if Twitter wishes to fight further (eventually to the top court) or stop and give in. It doesn't have to be written in the constitution for someone to be compelled to do something wacky in this country.
