How exactly it's "hilarious in hindsight"?

I'm not a KC fan and don't know anything about it, but i read here (on danbooru) that you can "marry" the ships. So if this was uploaded before they added the feature (don't know when it was), this was probably added to the pool because "predicted" the feature itself.

Or I'm missing something else that i don't know.

Yeah, that's pretty much it, I think. This was uploaded a few months before that feature was added.