Not surprised to see this particular retort again. Disappointed, to be sure, but not surprised.
You are, of course, correct, but the point is irrelevant. Clearly, rules do matter, or else systems would never be published. Certainly, we wouldn't be seeing "One D&D" if design quality didn't matter in any degree.
Good rules help make good DMs better. Crappy rules encourage or even empower crappy DMs. Mediocre rules can end up going either way--and I would certainly say that "no rules but what the DM says, no matter what the DM has previously said" would be in the "mediocre" category. This means that, yes, it is in fact possible for good rules to help against these problems--likewise, for good rules to help against problems with players. Nothing whatever can be an utterly impregnable defense against bad-faith behavior, but we can use tools that help. Rules can help us manage bad-faith behavior, call it out, isolate it, and address it. That's why we live in a nation of laws. Laws don't prevent bad behavior, but they give us a tool for addressing it, at least in part.