Are the posts and communities so strictly structured that a post cannot be a part of multiple communities?

That’s my understanding. If I understand correctly, a post belongs to a single community, but two posts referring to the same URL will be identified as such by Piefed, which is how the crossposts community consolidation happens.

In NodeBB categories and topics are all distinct elements, and the fact that a topic belongs in a category is contrived. A topic could be part of a user (pinned topics anyone?), a group (group only conversations?), or in this case… multiple categories.

Interesting, there’s definitely discussions to have about how to map that with the Piefed/Lemmy structure