That’s an interesting idea, but by the time this alleged impact happened, the Neandertals had vanished into the hybrid that became early Europeans.
The conventional view is that the original paper on this was invalid:
https://en.wikipedia.org/wiki/Chiemgau_impact_hypothesis
> This claim has been refuted by geological research and the finding of a soil horizon of undisturbed peat and sedimentation since the end of the last glaciation period.[1][2] The lake is in fact one of many kettles under the foothills of the Bavarian alps.
My view is, they found evidence of impact, but obviously they are way off on the dating — the impact(s) happened prior to glaciation.
Vanished into hybrid, yes, but did they vanish completely by then? It would seem to me that any pockets on non-mudblood Neanderthals would stay to themselves. Then with the Holocene extinction, their means of subsistence vanished, and so did they - except for our collective memory of dwarfs and gnomes. The absence of archaeological evidence does not mean they did not exist.