Which I find rather dumb, as it seems to me it undermines all the training he gone through but I digress and have to accept what Luke said.
I'd disagree. ROTJ makes a great point in stressing how potentially close Luke is to the Dark Side. Not in oblique ways but in ways that are notable enough. His actions during the duel with Vader become his final test. And he falters but then recovers.
Without a moment like this, the moment when he tosses down his lightsaber loses all narrative power. As it stands, that moment is the defining moment of his character and of the entire IPs philosophy and morality.