"Feeding" is overwhelmingly interpreted as "engaging with". I'm not engaging with the parent comment, I'm pointing out that they're blatantly violating the guidelines while avoiding engaging with their (non-)point.

This makes your point

> So, in a way, complaining about someone not following the guidelines is itself against the guidelines.

shaky at best, and invalid (at least according to the letter of the law) at worst.

I agree that the discourse would also be degraded if comment threads were filled with commentators constantly pointing out every little violation of the guidelines, which is why I try to avoid that, and downvote/flag/contact dang, but in egregious cases I believe that occasionally pointing out the trespass is valuable as a complement to those, because it explicitly reinforces the point that this kind of behavior is unwelcome and against the rules - a mere down voted/flagged submission may lead people (especially those from Reddit) to think that they're just being disagreed with.

(and, the difference between "people don't agree with my opinion" and "this behavior isn't appropriate" is that in the latter case people change their behavior or leave, while in the former case they keep their bad behavior but only let it out when they think they'll be agreed with, which further degrades discourse)