I doubt that this is the whole story, on account of females not having tusks. If you take the position that the tusk is an adaptation to improve feeding, which implies that it is sufficiently effective in that role for it to be selected for, then you have to explain why the females' lack of the tusk is not selectively disadvantageous.

My guess is that it was selected for a different reason (probably some form of sexual selection), and has proven to be somewhat useful in hunting. Both the possibility of sexual selection, and the possibility of features, selected for their utility in one use, having uses for other purposes, were important insights that allowed Darwin to defend his theory of natural selection against critics (they also help defend Darwinism against the claim that it is a fatuous tautology.)