The Lemmy project maintainers are communists, and quite possibly Marxist-Leninists, but I've never seen them make any statements denying genocide. They're been fairly explicitly against genocide, and have made statements directly to that effect. Meanwhile, the actual software has seen contributions from many people from all over the political spectrum. Because that's how the open source development community works.
Reddit's seen significant investment from Tencent, a Chinese company with well-known ties to the Chinese government and the CCP. Whether the people running the company believe China has committed genocide or not, those continued ties are implicit support for the government's actions (made even stronger by the Chinese government set to take ownership over a significant chunk of the company). They've been invested in Reddit for years, but that hasn't stopped basically anyone playing the "evil communist" card against the Lemmy devs from using Reddit. In fact, it's been used for a while now to try and convince people not to leave Reddit.
Reddit's also seen investment from white supremacist and actual fascist Peter Thiel. He invested almost a decade ago. Once again, no one seems to have any issue with the politics of the people backing Reddit.
The politics of the Lemmy devs can be separated from the usage of the software. The software is free, contains no ads, and usage of it does not directly financially support the Lemmy developers at all. Meanwhile, Reddit investors who implicitly deny or actively support genocide both in China and in North America stand to make considerable money off of farming our content out to chatbot developers and/or selling their stake in the business to greater fools.
Beehaw defederated from like 3 Lemmy instances. Most of us on the fediverse still have unfettered access to their communities. All it takes to not be disinvited from their party is to not shit all over their rugs.