This article doesn’t elaborate and I’m not convinced their source(s) are accurate. There is a competing “source” from another news organization that is claiming they caught this guy when his phone pinged a “smart device” (Alexa or similar) and its MAC address was logged.

However from other cases where genealogy databases are used, typically what happens is once a DNA profile is developed, it can be compared with other entries in the public databases. If there are family members that exist in the database, you can develop a family tree from the known person and attempt to identify the target from public records. Typically the more distant the relative is from the target, the harder it is to come up with a family tree.

AFAIK law enforcement uses publicly available databases and not the private Ancestry or 23andMe DBs, which are thought to be much larger.