Based off my own testing the answer is No, it's not cheaper.
Normal price for tonight is $53.81 for regular UberX or $50.19 with Wait&Save, when going from downtown Seattle to Seattle airport:
But if I schedule a ride for 4am in April, the price increases to $71.95:
I've tried times other than 4am too and they're even more expensive. You can try it yourself for your own destination but I'm pretty sure it would always be more expensive.
Also note that Uber doesn’t guarantee a driver will actually be available at the time you’ve pre-booked. I suspect that internally all they do is send out a ride request 10 minutes prior to your scheduled departure, without actually keeping a driver “on call” for your specific ride.
Uber doesn’t guarantee that a driver will accept your ride request. Your ride is confirmed once you receive your driver details.