Can this even be considered a free speech issue? They aren't deleting his tweet, only displaying it alongside a fact check. Of course you can try to call into question the impartiality of the fact check but that is a long way from not deciding not to show the content.
Not only does it not seem to be a free speech issue to me, from my point of view this is basically the best-case scenario for avoiding censorship of contentious issues on dominant platforms like Facebook/Twitter. It's the obvious conclusion to arguments like, "the way to deal with bad speech is with more good speech." Twitter saw speech they disagreed with, and they fixed it with more speech. They haven't censored an…
So wave the bloody shirt, that's AOK, but say that vote by mail facilitates fraud, and you get a personalized "We Don't Think So!" message from twitter.
Twitter hosts outrage mobs that have the stated goals of getting people fired, and it has caused people I was following to quit the platform.
They simultaneously want to exercise editorial discresion while not being liable for for all the outrageous or outright wrong speech they do host.