We might be able to answer the question better if you named the “other platforms” you’re referring to. It doesn’t seem like an unusual amount compared to, for instance, how much communist/transgender content Reddit had back when Reddit wasn’t as evil as it is now. (Who knows what Reddit’s like now. I haven’t been back since the two-day boycott over the API pricing.)
All that said, some of the communist content here is tankies. (That is, authoritarian communists who spout CCP or other authoritarian communist regimes’ propaganda.) Some of the Lemmy instances (like latte.isnot.coffe and lemmy.ml) are run by tankies.
That said, a lot of the communist content here is grass-roots anarcho-communist advocacy by people like me who ideologically lean that way.
I don’t think the lemmy.ml admins have been coy about it.
If you go to the lemmy.ml home page, at the bottom of the right column is a list of admins.
The first admin’s profile banner is a picture of Mao. And the second’s profile pic is a photo of Fidel Castro. The other two don’t have profile pics that are explicitly authoritarian communist and I haven’t had the patience to look through a whole lot of their posts or anything.
Just a couple of Reddit threads (via libreddit.hu) on the topic: one and two. Unfortunately what they link do doesn’t appear to be in the wayback machine as far as I’ve been able to tell.