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.
A fact check would be fine if it led to objective analyses of some sort, or even Wikipedia. But when I clicked it, it displayed some highly partisan sources, including a CNN article with its usual "Trump bad" vitriol. Maybe it was an algorithm's fault, but it didn't work at all.
How do you objectively report on trump without it painting him in a bad light?