I don't think the LLM was asked to check 10,000 files given these models' context windows. I suspect they went file by file too.

That's kind of the point - I think there's three scenarios here

a) this just the first time an LLM has done such a thorough minesweeping b) previous versions of Claude did not detect this bug (seems the least likely) c) Anthropic have done this several times, but the false positive rate was so high that they never checked it properly

Between a) and c) I don't have a high confidence either way to be honest.