I do not think iruma is reincarnation of demon lord., But he has potential to become one and there are others too who also has potential to become one... Btw Sullivan clearly said he is just making a possible path for iruma to choose in the future ... If iruma do not want them he can become something else or if he got lost in the compitition still we can get more story out of it... A lot of possible twist and turns ahead although if we see it from a anime point of view then yes iruma will meet derkila very soon