It's interesting that the paper doesn't mention this, but the answer here is Latter Day Saints. They have a very strong culture of genealogy and preserving family history. The study authors are all in Salt Lake City, which is the Mormon capital of the world.
Yes, I guess they assumed it went without saying.
I have a lot of respect for LDS genealogy efforts. It clearly goes beyond the baptizing the dead thing. Many of them take it very seriously, and they don't shy away from things that challenge the Mormon narratives (mainly DNA evidence not giving support for their peculiar American settlement theories).