Is she implying that "abnormal state" and "debuffs" are separate things here? Otherwise it seems odd for her to go "abnormal status is a failure frame, but my debuffs helped me kill that guy!"
I guess maybe it's a difference between specific effects like damage or paralysis, and her "debuff" just means "stat reduction", but that seems awfully specific if so. Usually poison/paralysis/etc are just seen as debuffs.
in most games they get grouped together, however paralysis/berserk/black-out would all normally fall under crowd control (CC), where as poison is under DOT. what she is mainly using is the most bare bone buffs and debuffs, hence she is thinking maybe MC's class [Debuffer] is a missed point. her only experience with it is him vs the goddess, a fight predetermined from lvl gap and age gap. after all it seems to be implied that she has been around for more than 1 round of hero summoning.
my hunch is that at one point the demon king had complete debuff immunity and gave his minions debuff resistance as well, this would render the debuffer only guy as a dead weight. however this only caused the goddess to sacrifice the debuffer early, and eventually lead to a near world wide removal of debuff immunity/resistance.
the other option is that his class was actually very strong vs all but the demon lord, and would be able to carry the class way to hard, giving a false sense of streangth. this would only end up hurting them right as they hit an debuff immune/resistant boss. so the goddess removed that possibility.