Your source sounds credible, so is my source.

It is common knowledge to English-speaking world that Mc, Mac, and O' all mean "son (or more generally, offspring) of". Thus McCain means son of Cain. MacDonald means son of Donald, O'Reilly means son of Reilly.

The real question, why the ancestors of MacCann decided to change to McCain, since Cain is not a new name and it has precise biblical origin in the early chatpers of Genesis? Even if MacCann meant son of warriors (hunters), McCain does not mean that all, it literally means son of Cain, no debate can change this simple reading. Uh, Cain is a hunter! Maybe that is why, dude.