>Is this thread-safe? Two refreshes in quick succession? Cloning a tab?

Yes I think so, the 'elem' it's dealing with here is unique, unusually 'pagedElementsCache->get' removes it from the cache (in a protected section) - so if another thread with matching owner+hint were to overlap, it would create another.
A new 'postfilter' is passed in for each call, so where it needs to call, no two threads would be sharing the same filter.