Yes and no.
You obviously need to scan TL heaps at some point. When doing so you'll have a set of root that allow you to scan shared/immutable heap. What is going on in the TL heap become irrelevant once you have the root. And getting the roots from the TL heap is another problem altogether (any kind of GC can be used for that, stop the world, where the world is thread local, or another concurrent GC, the important point being that it is an independent problem).
