Unless I'm mistaken, the Endeavor is the method of achieving CHIM, correct? By successfully following the Endeavor, you manage to control the tower. But the Psijic Order rejects the idea of the Endeavor. So at the same time, they're rejecting the concept of CHIM. They understand what CHIM is, the meaning of it, and Lorkhan's role in it, they just reject it.
Why? According to TIL's page on them, they (may) not wish to become gods themselves but merely wish to bestow others with this gift. But they reject CHIM. What is it about CHIM that they don't like? CHIM is a path to Amaranth. Do they reject Amaranth as well, or just CHIM as a way of getting there?
Edit: At the same time they want to stop the Thalmor. Do they reject going back to a pre-Mundus state, too, or do they think the Thalmor are doing it wrong?