Obvious hyperbole aside, I never said "never saw any concurrency testing". I said I never saw a place that was generally and reliably testing for concurrency bugs.

That is to say that following the standard practices at that company, an average developer introducing a concurrency bug will only tend to find out about that bug in production. And even if the developer wants to implement concurrency testing they will probably find it difficult enough to do that they'll give up because it's not a normal part of the test suite.

This is less true for people working on infrastructure like operating systems, and databases than it is for web developers.