@Gladiator02
Didn't they already say “Mirror will summon a demon that's possible for the user to beat”? They already know he's Op so just let him try
Yea, they're too easily distractable.
But maybe they thought the crack in the mirror broke it somehow. And that she's talking (and about a summon) makes them think it's all gone to hell.
(I thought the mirror didn't summon demons, it made illusions of demons, and this one looked like it walked through the mirror, so the thought might be that the mirror wasn't strong enough on its own to make a proper challenge, so it summoned a demon instead - and even if we know its a proper challenge, from their point of view it might have been the demon lord subverting the mirror for her own use.)
(Maybe I'm putting too much thought into a short joke manga