@gregger except bashubutts emptied the entire demon kings supply of mythril for his nuke bomb, magic edition weapon that the demon king implicitly then explicitly said to stop.
Daryl might not know this but Rizzeto couldve told him the full situation, that there's not even mythril to make magic lights for the Demon King's banquet.
It's also implied that it's not possible to just dismantle buttheadbaza bomb to get the mythril he already used, otherwise the Demon King at the leadt wouldve mentioned it in his reprimand of shithead asshole lanza.
So at the very least selling the mythril to the demons isnt going to put them on a war footing beyond what they already have, and since mythril seems like it has uses beyond just war (they remind me of copper wiring) the demons are going to have a domestic need to fill first before military, unless asshole buttsalot steps in and diverts them again and causes unrest in the rest of the demon king army, again.
Anyway it's totally possible to just handwave this ad the right thing to do, theres enough background to lead to this assumption.