What's the chance someone will come along with a classical analog that matches/beats this result? I remember seeing claims of quantum supremacy in the past and then clever people invented new classical algorithms that performed just as well.
There is certainly a chance, but for many of the researchers working on quantum computing (me included) that would still be a win: such classical results that beat quantum approached are usually incredibly beautiful and surprising.
However, on the extreme end, proving that quantum and classical computers are equivalent, would be probably more surprising than proving P=NP.
No, "quantum supremacy" is a technical term that has not been achieved before, and has not been claimed to be achieved by anyone serious. You may be misremembering previous claims called something different.
I guess you're right, I remember seeing quantum algorithms that were assumed to be more efficient than any classical algorithm and then people devised equally efficient classical algorithms. E.g. work by Ewin Tang https://arxiv.org/abs/1811.00414
They were definitely not being assumed to be faster. It was considered very uncertain. Quantum algorithms for which people are pretty confident there is no classical algorithm of similar speed, while still unproven, are Shor's algorithm (i.e., no fast classical factoring) and the present case of sampling for supremacy.
