It wasn't meant as a straw man, it was meant as a way to identify a boundary. The most restrictive regulation possible would be a total ban, and only a total and successful ban would prevent people from using guns in massacres. So "regulation" while well intentioned is practically meaningless. Those people would still be mentally ill, and outlawing guns or some lesser regulation would not have addressed the actual problem.

And yes, a knife in the situation is less dangerous. But the person is still mentally ill, a danger to themselves and those around them.

It's medical treatment that's needed, in the worst cases combined with confinement.

Gun regulation is not medical treatment. Gun regulation is a separate issue, admittedly related, just as knives and baseball bats are related.