Well, there is the 'perceived' deal that Mary made, and the fact that a demon like YED must know of Cthullhu, and while he may not like the Old God, he does have some heavy self-interest invested in not Cthullhu being allowed back into the world. Or perhaps Veidt, or something Veid does is directly related to a possible seal.

There are ways to work things in, just remember that everything the YED does it is done in the name of self-interest.