@Owyn in chapter 9 page 7 Kai asked Abyss if what Ren used was magic and Abyss explained that it was not because what Ren was doing didn't use the mana in the air which is what magicians of their world use. This basically led Kai to believe that her Ren's powers deviate from the laws of the universe similar to Tenohira, which should be given since they both originate from his blessing.
Also the reason that others couldn't use magic in Tenohira was because mana didn't exist there, but after that magician merc got brought there he gathered all the remaining mana in his body to cast a pitiful spell which led to the creation of mana in the atmosphere of Tenohira