Both could be programmed. Someone could also copy the Lemmy API and use arbitrary Lemmy apps with Piefed.

This seems like an interesting idea. On one hand, I could see how it could hamper development, but on the other hand, it would be nice if all of the threadiverse platforms (Lemmy, Piefed, Sublinks, Mbin?) were standardized enough that the apps could be interoperable. I think giving users multiple options for how to access and interact with the content would be good for the fediverse as a whole.