I suppose that nothing specifies that the memory backing non shared data blocks cannot be accessed by concurrent agents, but pretty much all spec algorithms assume there is no concurrent agent access.

The thing is Moddable actually does have multiple agents running on the same Realm, but the only shared objects are deeply and transitively immutable, so any mutable memory is still per agent and not shared.

I think we should have an explicit invariant similar to the liveness one that defined observable garbage collection so that a host can only create agents such that a program in a single agent can only observe an execution as if there was no other agent running concurrently (notwithstanding shared array buffer)