For me, Comcast only allowed a few models. Because it requires full control. They uploaded a Comcast firmware on it. And they warned me that after that the device is basically stuck to only work with Comcast. This was around 2017.

That sounds odd - I wonder if it was a relic of your local infrastructure. I used my own DOCSIS modem with its own firmware on Comcast around the same time. It was a generic Arris Surfboard.

Comcast's list of approved devices includes (or used to at least) some of the more common Arris Surfboards; I also used one when I had Comcast service. You probably also used one that was 'compatible'.

The list of compatible devices was simply a list of all devices that supported DOCSIS 3.1 at the time.

At no point was my firmware swapped out, or anything of that nature.