I think you will need to take the monorail to the TTC and go from there. That's how we got to MK from MGM in February...took the TTC bus from MGM and then hopped on the monorail...didn't take a lot of time to do this. Oh wait...we took the boat and not the monorail, but same difference.
Yes, that is how it works. The busses that go to the resorts are up near the main entrance to MK, but the busses that go to the other parks are located at the TTC, so you either have to take the monorail or the ferry to get to them.