The problem is that "sustainable" is always based on past observations. There were 40+ years of observations for these crab and a stable fishery. But if the environment changes for the worse, the data-based definition of "sustainable" for a stock might not change fast enough to compensate.
So the "new sustainability" under climate change has to be much more precautionary than before, and yet not shut down on false signals. It's tricky science even when intentions are good.
They also disagree with you that they've never been sustainably fished.