He's making a point about responsibility/liability.
If you only get copyright for the prompt you make, but not the output, then it's like being responsible only for the prompt, but not the output.
Ie he's only responsible for pushing the boulder up the hill. The fact that it rolled down from the hill and crushed someone's house "isn't his fault" (he doesn't get copyright on it).
Well, you are responsible for the consequences. Liability is simply a different thing than copyright.
That is not how responsibility works anywhere. If you are stealing a gun and murder someone with that gun, you are still responsible, even if it is not your gun.