The other issue is that as Lemmy posts don’t have unique URLs that can be used by every instance, you can’t just have a megathread with links to discussion threads, as that would work only for one instance

I’m curious as to how Lemmy apps solve this issue (which they seem to have done), and why the web interface doesn’t have similar functionality. In Eternity and Thunder, I can tap on a link to a post on another instance, and it opens the corresponding post on my home instance.

Is there some lookup table under the hood to match post IDs between instances? Whatever system apps are using, why would this not work more broadly?