Not totally true, if you can robustly express your tiny change as a `sed` or `awk` script, you can just append to the generator pipeline. Speaking from experience, do not condone, etc.
Modifying the generator itself to do something different every time, and doing GP's stated "regenerate and throw away the old stuff" is in line
Which is itself a generator
Since the issue is the maintenance of such systems, it is absolutely relevant.
However, if your awk script requires the current state of the generated code as input in addition to the output of the black-box generator, and tries to reconcile a diff between the two things, then yep, I consider that busted.