You can't take the science in this show seriously. Janeway turned into a salamander and back in one episode.
Also, the Augment virus. Who knows what crazy remedies and cures the Empire has tried over the past 100 years to reverse the effects. And even if the Augment virus isn't connected to these particular Klingons in any direct way they could be a subgroup of Klingons that just happens to look different. They ARE a vast interstellar empire, after all. Not every Klingon will look like the other nor even similar in some cases.