I think it's popular because it's easy / free to set up and the friction is very low for new user, not because it's actually any good.

I think it's actually horrendous for things you want to use forums for. I don't remember using IRC in lieu of forums (did we?), why do we use Discord for the same?

They choose it because its engaging.

Discord is really good at sucking you into conversation and capturing your attention/time. Its way better than IRC ever was. But that doesn't mean its a good format for information, issue threads, work discussion and such.

What in the UI or UX of discords has this effect?

Presence indicators, activity indicators, emoji feedback, and video chat, to name a few. Features that provide instant gratification.

I guess endless frustration and wandering is engagement.

Those insights are things I've tried to bake into Booklet - for instance, a really generous free plan and no per-user pricing. Existing community platforms (like Circle) are not set up for bottoms-up adoption, so it ends up feeling like gross enterprise software.

If Discord hijacked all the communities, wouldn't elixirforum not exist? Seems like a contradiction...

I am very sure they weren't speaking literally...