It's not modifying the generator. The generator may be a proprietary black box. It's wrapping the generator in a bash script that pipes the result through AWK, etc.
As other commenters have noted, if the awk script is just a pure function of the output of the black-box generator to a new output, then I would consider this a modification to the generator, and no problemo.
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.
Sure, that's orthogonal. If you wrap the generator in your build system and still always regenerate, it's effectively the same. And also, I think, not what GP was talking about
Pedantic. There's a world of difference between grokking a new code generation DSL+codebase and a shell one-liner that fixes a string that is obviously invalid.
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.