When free speech on the internet is discussed, it is within the context of freedom from excessive moderation on a platform. To call a Fediverse a "platform" doesn't seem quite analogous, these communities are more like their own platforms that are far more intensely moderated than even Twitter or Reddit.

Free speech advocates who aren't convinced by the "just go to another platform" argument in a discussion about Reddit or Twitter censorship likely won't think the argument is worth accepting simply by redefining what a platform is.