I think that's exactly what it is - precisely because they are (assumed to be) better channelers than those of the Third Age, in his arrogance Demandred relates that to being better in EVERY aspect, not just in skill with the Power.
(Edit) He doesn't just do it with Lan - he struggles with...
That was my assumption, as well - the rest of the Chosen were, from Demandred's perspective, solidly accounted for, so Asmodean was the one that came to mind first in his moment of shock at fighting someone so skilled. Obviously this was shot down quickly, but this line of thought makes sense...