It's a property of the "semi-artificial" problem chosen by Google. If anything, it means that we should heavily discount this claim of "quantum advantage", especially in the light of inherent probabilistic nature of quantum computations.
Note that the OP wrote "you MUST compute something that is impossible to do with a traditional computer". I demonstrated a simple counter-example to this statement: you CAN demonstrate forward progress by factorizing big numbers, but the problem is that no one can do it despite billions of investments.
If they can't, then is it really quantum supremacy?
They claimed it last time in 2019 with Sycamore, which could perform in 200 seconds a calculation that Google claimed would take a classical supercomputer 10,000 years.
That was debunked when a team of scientists replicated the same thing on an ordinary computer in 15 hours with a large number of GPUs. Scott Aaronson said that on a supercomputer, the same technique would have solved the problem in seconds.[1]
So if they now come up with another problem which they say cannot even be verified by a classical computer and uses it to claim quantum advantage, then it is right to be suspicious of that claim.
> 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.
Note that the OP wrote "you MUST compute something that is impossible to do with a traditional computer". I demonstrated a simple counter-example to this statement: you CAN demonstrate forward progress by factorizing big numbers, but the problem is that no one can do it despite billions of investments.