Wing thumbs allow the wings to have some cling. Not like static. Wing cling. Useful for hugs of death (breath?) in battle.

The main source of most dragon wing 'knowledge' is bats, where the wings are two of the four limbs and need the thumbs for proper grippy navigation. In Dracthyr, one could say that the wing thumbs are holdouts from protodrake wings, where there are four limbs and the wings are used for pedal locomotion.

What about that random digit coming from the elbow joint of the wing?

That's the wing """thumb""", we're in agreement. Unclear in the text. That thing is so bulky for no reason, and none of the other dragons have it. So Blizzard-Neltharion did it intentionally. But whyyy

Oh, the thumb is normally the one sticking out at the wrist joint, which would be at the top of the wing in your image. The one at the elbow joint - closest to the cloth drape on the wing - is just weeeeeird and shows somebody at Bliz has no idea how wing anatomy works. :\

Exactly! It makes the wings bulkier and only the dracthyr have ut