It's convoluted and not explicitly explained, but I think what happened is that Mo Wei's teacher/master made him promise something about not training someone in Trillion Blows Art, but then Mo Wei is trying to pass it on via a loop-hole somehow, which is why Noh Gajang's understanding of his own strength, qi and Murim are so skewed, because Mo Wei intentionally misled him. Noya seems to be continuing the deception while also trying to aid Noh's training (not that I can actually tell what he's doing, but it may have something to do with blood, veins or impurities?) to respect Mo Wei's wishes.
Also I think it's possible that Noya here (Chun Myunghoon iirc) killed Mo Wei in some kind of mindless berserk mode, and didn't remember.
Both of those are just speculation, of course.