It depends... If STR was being used for encumberance, you could make the Kerrblade so heavy that only some (super-strong) Alien types could use it. Or give some Alien races a very high Close stat, while the blade has a low to-hit probability (is that even being done right now? seems there's no place in weapons_melee.ufo to say that a Knife is more accurate than a Kerrblade, for example...)
Or you could make Alien inventory screens different from Human, so that while a Kerrblade takes up a hand or backpack slot on a Human, it can be kept in an Alien's belt or whatever. So, it is less useful for Humans as it precludes them from carrying other weapons.
But, basically, I don't know for sure yet, and I suspect that is just a bit of "flavour text" in the UFOPedia