Kasparov complained that Deep Blue had access to a myriad of his historical games, so it could prepare purposely just against one opponent. At the same time, Kasparov hadn't any access to train against Deep Blue's way of playing.
Stockfish had a similar disadvantage against Alphazero. With LC0 being more openly available, it can start improving its heuristics against this type of opponents.
I agree with you in general, but to be perfectly fair, even the Zero-style AIs have a few hyperparameters such as block size and CPUCT value etc that can be tuned to produce engines that play differently. And if Stockfish is the measuring stick by which you gauge progress, you'll tune those values to produce good anti-stockfish nets.
That said, I think the effect here is probably negligible.
Except that LC0 wins decisively against all other engines too. You just hear about the wins vs. Stockfish because Stockfish is the best conventional engine so that's more newsworthy.
Sure, but that doesn't really change my point, only the dates involved. You can debate how fair the Kasparov - Deep Blue match was, but even being maximally generous, by the mid 2000s it was completely impossible for a human to beat a computer in a serious, fair match.
Stockfish had a similar disadvantage against Alphazero. With LC0 being more openly available, it can start improving its heuristics against this type of opponents.