I love raspberry pi but they are definitely not as "open" and drop-in compatible as most people think.
That seems to be a problem with most ARM devices, so may not be specific or the fault of the Pi people.
So, yes, 'non-Debian' which (Debian) includes Raspbian, not 'non-Raspbian' which (Raspbian) precludes Debian.
Raspbian is probably closer to Debian than Ubuntu is, but it's still quite clear that you're not on Debian.
https://www.debian.org/derivatives/ https://wiki.debian.org/Derivatives
But it doesn’t surprise me that it just works since Ubuntu is a commercial distro. They even sell products for the Raspberry Pi version.
I used to use Ubuntu MATE on pi, but when the Pi4 came out it took a long time before it was supported.