"I bet ROL and ROOL haven't come to a collective decision about all of that."
I suspect you are right, and the announcement reflects what's been done so far - that they've come to an agreement. There are, I imagine, still many things to work out. Ensuring no future API incompatibilities should be a piece of cake; just implement a formal procedure for API "allocations" which could possibly be based on (or an extension to) the present resource allocation system; it depends on how it's handled internally, which I don't know, and precisely what needs to be covered and how detailed it needs to be. I guess that if it was just the two parties things would be relatively simple, but with the shared-source initiative and the fact that means any of us can contribute, it complicate things a touch.
For differences that have already been committed to Userland things will be more difficult to resolve.