“My understanding is that if someone in your extended family has ever used a commercial genetic testing service, that’s enough of a DNA trail for authorities to zero in on a suspect.”
That’s not how the process works. From what I understand from LEO who do this stuff for a living, Ancestry.com, 23 and Me, etc., don’t just turn over its DNA database to LEO with without a warrant, and a warrant will only be issued if LEO already has a suspect. More likely, Kohberger or a close family member voluntarily uploaded DNA sequences to a public site where the DNA is available to anyone who asks.
With that said, the big lead was the tip about the white Hyundai Electra. LEO probably learned from the WSU campus police that an on-campus parking permit was issued to Kohberger, who had a graduate student and office on campus, and from there LEO took a DNA sample from a coffee cup, soda can, etc.
That certainly isn't how they identified and caught Joseph DeAngelo the ex-cop in California who and raped or killed over two dozen women.
Check back with your LEO buddies and ask them specifically about the DeAngelo case where DNA evidence from a cold case was used to produce a close DNA match with one of Joe DeAngelo's relatives before they even knew who the suspect was.