Is it true, what he said, about Tamriel being Mehrunes Dagon's old plane of Oblivion or whatever?

He could be. There has been a discussion about it, just of the first page. If you have specific questions, please ask them there.

http://www.gamesas.com/index.php?/topic/1255204-was-mankar-camoran-right/

There's no true or false when it comes to TES lore, only conjecture with varying degrees of accuracy.