I don't think that "Code you did not write is code you do not understand" implies that the only way to understand code is to write it. I can read code, run it, debug it and figure out how it works.
It's true that all of that is possible with AI generated code. But the thing is, is it worth for me spending time understanding that PR, when I can make the change or launch my own agent to prompt it myself? At least with my own prompt I know exactly what I want it to be.