Its very hard to keep up with what supports what.
They may or may not be the case, but nobody is even trying. Support the base protocol for other platforms and make specific hacks for each one to fix differences. I would at the very least expect read-only support, with some banner that says "this post is from platform X and comments are currently not supported".
I keep hearing excuses like "PeerTube can't link with Lemmy, because then it would have to download the video files", instead of just implementing the obvious solution of NOT downloading the video files and let the parent platform handle it. There's too many purists that try to implement the perfect solution and say it's impossible because it doesn't handle the edge cases.
Even Reddit from years ago spent a lot of effort making sure thumbnails were extracted from many many different platforms, whenever you submitted a post. It didn't matter that there were a lot of different video or news platforms. It worked for almost all of the cases, and if it didn't, it got fixed pretty fast. No shared protocol for that, just hardcoded lists of domains and URLs.