> If they can't, then is it really quantum supremacy?
Yes, quantum supremacy on an artificial problem is quantum supremacy (even if it's "this quantum computer can simulate itself faster than a classical computer"). Quantum supremacy on problems that are easy to verify would of course be nicer, but unfortunately not all problems happen to have an easy verification.
Yes, quantum supremacy on an artificial problem is quantum supremacy (even if it's "this quantum computer can simulate itself faster than a classical computer"). Quantum supremacy on problems that are easy to verify would of course be nicer, but unfortunately not all problems happen to have an easy verification.