Imagine a future president picks a Catholic forum to make the same sort of announcements that Trump currently does, specifically a catholic forum that bans any advocacy of pro-choice discussion. It is relatively easy to find a similar forum on any side of a modern hot button political issue.
Twitter's upcoming option to limit replies is being touted as a politician's dream, but in the US it's likely going to be unuseable for the same reason
If you're going to make legal nitpicks, you should probably have a thorough understanding of the jurisprudence.
And I wish you were right. That was how I always read it too but we've found out repeatedly - and recently - that state & local governments (and state universities) can ban numerous things, contrary to the 1st Amendment.
Considering they could log out (or open a private tab) and view the content, obviously it wasn't access to the information that was fundamental but the act of the President taking a step to reduce someone's access.
With that in mind, the underlying host taking a similar action is either a) Okay because it's their system? or b) Bad because they're blocking or altering the message?
We're in this really weird spot of free speech vs private property vs public forum vs free access vs..