Love you leaving out the orthodox part of quote. But it’s also something known and being researched in many groups that tend to stay in their own groups for relationships and children.

https://vcresearch.berkeley.edu/news/bottlenecks-reduced-genetic-diversity-were-common-throughout-human-history

https://old.reddit.com/r/Jewish/comments/1cepo3s/ashkenazi_jews_and_inbreeding/