Here's what Michael Kirkbride, the developer who wrote that speech, had to say about its truthfulness:
But its no fun to rely on appeals to authority to judge something like that. Lets get a little more creative.
Is it really that hard to believe Mankar's version of the story? The traditional version of the story has it that Lorkhan created the world but that the other gods killed him for tricking them. It doesn't even have to be a different version of events, it could even be the same version of events form a different perspective. Lorkhan (who is padomaic, meaning its not a huge stretch to think he could be a Daedra) got other et'ada to help him construct the world. Then the Aedric et'ada turned on him and killed him.
Combine this story with the Aldugada, which claims that Nirn is made up of bits of stolen kalpas horded by Mehrunes Dagon (leaper demon king) and Lorkhan (the Greedy Man.) That explains why, with Lorkhan gone, Dagon views Tamriel as "his birth right." He helped create it.