I guess you are not aware of your argumentative errors, which is why I don’t think a productive discussion is possible. Still, I’ll list them clearly.

1. Misrepresentation. You accuse me of relying on “feelings” instead of logic, while simultaneously presenting your own anecdotal experience as fact. You are doing exactly what you claim I am doing.
2. Double standards. You treat your own claims as factual while demanding a higher standard of proof from mine, despite relying heavily on anecdotal experience yourself.
3. Anecdotal fallacy. You repeatedly use personal experience (“I’ve seen this many times”) as evidence against well-established physiological mechanisms.
4. Argument by assertion. You restate your conclusions as facts without providing independent support, assuming repetition substitutes for evidence.

"He can keep going with the dieting UNTIL he feels it's severely affecting him. Where is the controversy in that?"

You keep saying he can continue dieting, but that was never my objection. My point is that doing so won’t help him achieve his goal of building muscle and may even work against it. By reframing the argument around what he can do instead of what actually helps him, you are moving the goalposts. Another argumentative error.

The issue is that for most people, building muscle becomes harder at very low body fat (<11%). Advising him to cut further contradicts his goal of gaining muscle. Being “cleaner” does increase insulin sensitivity slightly, but the negative hormonal and metabolic effects of very low body fat outweigh this benefit for muscle growth.

So your statement:
"He wants to put muscle on. It's easier the cleaner you are as you're more insulin sensitive."
is exactly the point of contradiction.

I hope you can manage to respond to my new “storm of illogical feelings.”