That kind of legal ass-covering is expedient when you are going to explicitly reproduce someone else’s source-available work. It’s cheaper in that case to go through the whole clean room hassle than to risk getting into an intractable argument in court about how your code that does exactly the same thing as someone else’s code came to resemble the other people’s code so much.

But, for the general case, the argument still stands. I have looked at GPL code before. I might have even learned something from it. Is my brain infected? Am I required by law to license everything I ever make as GPL for the remainder of my days?

Yes, it will sometimes depend on the unique qualities of the code. For instance, if you learned a new sorting algorithm from a C repo, and then wrote a comparable imperative solution in OCaml, that might be a derivative work. But if you wrote a purely functional equivalent of that algorithm, I don't think that could be considered a derivative work.