That's just a cover. He did defeat his demon lord after she trained him to be able to do so, but rejected killing her outright and instead broke her "demon lord" status and it's in-built compulsion to destroy everything.
Yes, technically, and for technicality.
Technically MC didnt kill the demon lord even though he clearly has the power to do so. In the first chaps, instead of killing demon lord he stripped the status itself, resulting the demon lord still alive. And for this reason the MC is in group who "didn't kill" demon lord
Ugh the story seriously downplays the 30000 year training part and how OP the MC should be.
He defeated someone who even the gods were forced to seal, my ass. He literally has power to destroy worlds(obv cause all the DL powers are his) but you're telling me he couldn't sense the teleportation guy coming, couldn't react fast enough to intercept his skill usage and couldn't even instantly use his own skill? His 30,000 year training should've made everything a muscle reflex... Ugh such poor writing. All for the sake of this shitty plot. Like what's even the end goal? He keeps getting pushed around...
Would've been better if instead of the generic battle manga this would've been about romance b/w the MC and the FMC...