If you were talking about Apple or Google, you might have a point, although you would be better off to talk about "freedom of speech", since "the First Amendment" refers to a very specific piece of legal text (and one that nobody has actually ever said should apply to app stores).

But we're talking about F-Droid here. It has a tiny market share. It uses software that lets literally anybody else set up a repository and have it appear integrated in the same app. It doesn't benefit from any of the legal or social advantages given to large corporations. F-Droid, unlike Apple or Google, can't in fact do much to limit anybody's speech.