Not sure about transgender posts, but the communist posts are probably because other platforms are owned by huge corporations, and huge corporations generally don't like communism. Lemmy can never be bought by a huge corporation (at least not entirely).
If you don't understand why that is not true of things like lemmy, you do not understand lemmy
lemmy cannot be bought in the same way that Linux cannot be bought. You may buy an instance (or a distro) but that doesn't (and can't) get you the whole lot. Anyone could start a new instance, thwarting your bid to collect all the users
Wikipedia tells me there are many non commercial implementations of XMPP, alongside commercial products that also use the standard. I don't see that as a good example of corporate takeover of the whole thing