I agree that it definitely can't be completely without cost. But I'll speculate that they may have arranged for the don't-care bits in the pointers to all be 0 when GC is not running and doesn't need the bits. If so, that could mitigate the TLB waste during periods when GC isn't running.

In other words, just because the alternate mappings exist doesn't mean the pointers always have values that actually use them.