Putting it the way you did is very misleading for folks not familiar with the difference between complexity and computability.

Quantum computing is not about computability, because it is well agreed that a quantum computer can solve exactly all problems a classical computer can and vice versa (Church–Turing thesis).

What is not agreed on is if there are problems that a quantum computer can solve more efficiently than a classical computer (where more efficiently means either in time or space and has a strict mathematical definition).

Now, put simply, if someone could come up with a computer that would solve a task faster than the theoretical limit of a classical computer that would put an end to the discussion once and for all. This is what Quantum Supremacy means in the article, or in the words of Scott Aaronson:

> But what quantum supremacy means to me, is demonstrating a quantum speedup for some task as confidently as possible.