Yeah, disintegrating a frigate with heat comparable to the outer layers of a star is ****ing miles ahead of anything Malgus has done in the force dude. Vader alone has crumpled TIE fighters mid flight, disintegrated Lyleks, manipulated freighters, collapsed massive platforms, melted durasteel/blown a hole through it, collapsed massive mining platforms, collapsed Wookie huts off a Wroshyr tree, etc. Malgus is less powerful than either of them, and less skilled than Vader overall.
I agree he'd lose a duel to Malgus, but he could overwhelm him with telekinesis once he realizes he'll fall in a duel.