Quantum supremacy is the idea that a quantum computer could compute something much faster than a classical computer. Aaronson's big idea was that a quantum computer could just sample outputs from a complicated quantum random number generator, and that would pass as quantum supremacy if the classical computer could not simulate it efficiently.
That is what Google did two years ago, and Aaronson was the journal referee who approved the claim of quantum supremacy.
Now some Chinese researchers have shown that they can simulate Google's output on a classical computer. Aaronson says the Google team claims that they can keep changing the benchmark until they find one that the Chinese cannot simulate. He does not believe them.
Aaronson does not go as far as saying that Google's quantum supremacy is all a big hoax, but that's what I get out of his post. Read it yourself.
I am waiting for the quantum computers to compute something that is demonstrably difficult. That has not happened, and may never happen.