The Pixel watch supported carrier list also makes no sense, because for some reason the new carriers for the watch 2 don’t support the first version?
They often don’t update the list for older devices. The supported list is whatever it was when the device came out.
I actually don’t know what you mean by “Google’s allowlist” but the underlying carrier might matter more than whatever small “carrier” you want to go with. E.g. Telus, Bell, Rogers(/Shaw), Videotron basically run all of the cell networks and then lease them out to the small carriers (which they actually either own, or will eventually).
So if those big guys are on the list, you’re probably fine (but I don’t actually know how it works). You can google “{name of carrier} mobile network” to find out whose network(s) they’re on.