I took GPs meaning that the QA person in question sucked, but them being the best meant the other QA folks they've worked with were even worse.

That's not at all what they meant. They meant they ended up raising their own quality bar tremendously because the QA person represented a ~P5 user, not a P50 or P95 user, and had to design around misuse & sad path instead of happy path, and doing so is actually a good quality in a QA.

Let's call the person in question Alex. Having to make every new feature Alex-proof made all of the engineers better.

It's possible but I'd guess they are probably not worse than the average user.