Just from reading I would say he's using Quad9 as upstream DNS for his PiHole (which is used by clients)
I know that. I mean, what is the service Quad9 doing.
It's another "privacy" friendly DNS driven by IBM, Packet Clearing House (PCH) and Global Cyber Alliance (GCA). Placed as an alternative to Google's DNS
"Privacy friendly" is what we are worried about. It's from the US gov't so we really don't trust it.
I'm sure you've noticed the quotes around "privacy".