I wonder how they knew whether this is just one narwhal's or one colony's modified behavior or the behavior of all narwhals'. Sea mammals are pretty intelligent, who's to say that this one or this colony hasn't adapted some use for their task that is restricted to just them?
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.)
I am not suggesting that there is competition between males and females - both are in competition with the rest of their world, especially with their prey and others seeking the same prey. If one posits that the tusk was sufficiently advantageous in helping males feed that it was selected for, that seems to imply a corresponding disadvantage for tuskless females in their competition with the rest of the world. If the female ancestors could feed just as well as the males, then what was the source of the selective pressure on males alone? Perhaps the males' sexual competition between themselves demands more food or leaves less time to find it?
