Yes, that's basically what those schemas are about. You can create different schemas for different kinds of posts and content structures, so something like Lemmy should be possible. The Fediverse has something similar as well, but the way you introduce new schemas is different between the two as far as I understand it. In the former you'll have to adapt some features of the underlying ActivityPub protocol to your new usecase, or work with others towards extending the protocol. The later allows you to just declare and describe your new structure in a machine-readable way, and others can then choose to support it. So Bluesky is more flexible and open in that regard, but could also end up more fragmented.