They are big so they don't care. If it was Threads that had some federation problems it would be a high priority task for the developer team to fix it. But Lemmy is too small for them to care about us.

I doubt they will ever add it at this point. Mastodon can display markdown on posts from Akkoma just fine so it must be that they don't want Masto users to have access to markdown at all. Fortunately, Fediverse is more than just Mastodon so you can join an Akkoma instance instead, or one of the forks of Mastodon.

Perhaps some client apps don't do a great job of showing a small teaser when a post is long - I've had some negative pushback from Mastodon users when my Wordpress blog uses the ActivityPub plugin to share it's content onto Mastodon. They talk of needing to scroll lots to get past the post, which sounds like a badly made app to me.