The thinking of the Blessed opponents was partially right. In order to summon this large amount of imps, Noah had spent much of his energy.
Something that had not been specified when the skill leveled up that Noah had found out when he summoned the first imp was that the "price" per summons had dropped quite a lot. As the amount of imps he could summon increased, the amount of energy he used to summon the imps decreased.
This was great news for him, since if the amount of energy used to summon the imps was still the same, the ability to level up would be almost useless since he wouldn't even have enough energy to summon as many imps as the ability allowed.
But the Blessed Ones were right about one thing: after summoning another 20 imps Noah had run out of energy. This time he even prevented the imps from eating the meat of these humans, as he would use it to replenish his own energy and earn some stat points.