So what if Lemmy, Piefed, Mbin, and NodeBB made it so that only the first matching community gets the post?

I'm pretty sure Mbin already does that with sorting posts into communities based on their hashtags. Does it not do it with mentions too? I can't really test it since 99% of federated posts only mention one community, if any. So I'm struggling to find a post that mentions two communities, let alone two that are active enough on my instance to compare them.

But like, is it actually an issue? I always get the impression Lemmy users have more of a problem with the hashtags and mentions in general, not with the fact the post appears in multiple communities. Which would be easily solved by having their instance remove those from microblog posts.

We can already tell which posts come from threadiverse software and which don't (because we use audience, Mastodon doesn't.)

I honestly don't think that's a good way to decide between threadiverse and others in general. There's no guarantee non-threadiverse software won't make use of it in the future.