Just no, that's not how any of that works.
I guess that lie is convenient to legitimate the lawsuit.
The output of stable diffusion isn't possible without first examining millions of copyrighted images
Then the suit looks a little more solid, because (as you pointed out) it isn't possible for the stable diffusion owner to know which of those copyright images had clauses that prevents stable diffusion trading and similar usage.
The whole problem goes away once artists and photographers starting using a license that explicitly removes any use of the work as training data for any automated training.