Maybe true with endpoints in the actor object, but clients should be able to refresh any object from its URI at any time, I believe this is a spec requirement rather than just advised, although I could be wrong.
As for Activity Pub lacking clarity, I do completely agree, it's going to be years before we have a good understanding of what the spec actually necessitates in production implementations.
This would be "interoperable" in that it's easy to go from one framework/software to the other, but it's still not transparent. The current way of doing things is to use the url of the object in the search bar, and thus use the search bar as the "new" url bar. In practice it is more interoperable because all softwares then behave the same.
As for Activity Pub lacking clarity, I do completely agree, it's going to be years before we have a good understanding of what the spec actually necessitates in production implementations.