This is just a guess, I might have some availability bias (or whatever it’s called, not gonna bother looking it up), but I think a lot of the hate stems from how inaccessible content in Discord is without having access to the server or a Discord account. It’s not indexed by search engines. So many projects use it (because it’s so easy to make a server and get it going) and post documentation, issues, etc. exclusively there.

The Lemmy community is not very diverse yet. There are a lot of tech enthusiasts, FLOSS enthusiasts, privacy enthusiasts, and leftists. Most of these groups of people have reasons to dislike Discord. Tech folks hate it because of what I said above. FLOSS enthusiasts hate it because it’s not FLOSS. Privacy enthusiasts hate it because it’s not end to end encrypted. Leftists hate it because it’s it’s a company.

So while everything you’re saying is true, I think it’s that most of the types of people who use Lemmy have various reasons to dislike it.