It isn't out of the question that groups could have moved quite a bit since there is evidence of turnover in either Caucasus or Western Europe later on, but I am not sure coming from somewhere else solves the puzzle. Existing evidence suggests that the MRCA of known late Neanderthals including those predating Thorin was in Europe [2].
So Thorin's lineage could have traveled from somewhere without other hominids and beelined for this site ten years prior, but it is not very parsimonious considering he was in a layer with the same PNII style artifacts for thousands of years before and after him? However, the PNII artifacts don't appear to be rooted in the previous ones of the region so perhaps there was an older exotic origin.
[1] https://www.nature.com/articles/nature26151 [2] https://www.science.org/doi/full/10.1126/sciadv.aaw5873
there is no evidence that they were near to each other for 50K years, only that their genomes diverged 50k years ago which was the last time they were near each other.
At the separate-times-and-places that they died in "France", the recent migrants may only have arrived in the last few months, having spent the previous 50K years over in that other far away place, and died only somewhat near each other a decade apart
the mixed genome of early modern humans and the lack of mixing of Neanderthals is a separate piece of evidence, and it may point to Neanderthals not being as sexy-social, but that's just a "may".