The so-called god Malak comes out of the blue, never having been seen or heard of before. And the system he uses to control the river water appears to have been in place for some time. The Sleestak, including Enik, accept him immediately as a god without question or even consulting with their library of skulls.
Apparently, Will and Holly had forgotten the events of The Pylon Express (1975). When the three moons line up in the sky, sunrise opens the door to the keyless pylon, bringing visitors on a tour of various worlds including Earth.
Malak's use of the English language is a little too advanced for a Cro-Magnon.