Yeah, I've just read an article the other day, saying that while on paper it's well-proven quantum computers are far more capable for certain calculations than the current tech, there's still no practical proof of that; something about the need for more qubits, but when they go for it, the entanglement goes out of whack. And maybe google has managed to create a proof.
But on the subject of "google claimed" and "quantum supremacy" - well, it's a lot of hype, it seems google did not claim anything as such, only a google employee said something to this effect while a reporter was in hearing distance.
Apparently they have built something which does calculate one very specific scientific thing well & quick.