Isn't tamriel technically the daedric realm of Lorkhan?
According to Mankar it is, but he also gets other things wrong while he's speaking. He's a fanatic that believes having the world being destroyed is a good thing, and that torturing his loyal servants is good. So you can't exactly listen to everything he has to say.
Other beliefs say that Lorkhan convinced the gods to create the mortal realm of Mundus, and then that drained their power and they were left weakened, which is an explanation as to why all of them wouldn't jump down and save everybody all the time.
Other hypotheses range from Lorkhan being a Daedra, to him tricking the Nine Divines into making the Mortal Realm, to him giving all of his power to create the realm.
From the elven viewpoint, Lorkhan tricked the lesser spirits (The Ehlnofey) into becoming mortal, which eventually became the Aldmer, which became all of the other elves. While men believe Lorkhan created them from nothing, and think he doesn't deserve the bad rep he's gotten.