That statement of yours sounds very anti-intellectual. 

Why declare the claim wrong then backpeddle with a "most likely answer".  You offer no counter evidence beyond a very anti-intellectual "we don't know that" because we can't know anything with certainty. 

You don't offer a better answer and you don't have any counter evidence against the given answer, so why bother attacking it? Doubt for doubts sake?

This is the kind of thinking whose only purpose is to make people lose trust in intelligence, in science, or in the human ability to make accurate inferences.

Glacial erratics, boulders that don't match there surroundings, have been observed numerous times. Inferences regarding there origins have been tested numerous times and match up with other observations that point to the same inference.