Nice. I wasn’t aware they managed to assemble all parts with Linux drivers. From all I read (not an expert at all) that seems quite tough. What remains is a runtime environment/infrastructure for everyone who’d like to run Android binaries/packages on the phone.
SFOS is not an Android fork. As many classical Linux distros, it controls sessions through systemd, the compositor is Wayland and the standard c lib is glibc. However, compatibility with the drivers of many hardware vendors and certainly also running binaries build for Android require Android libraries and abstraction layers to be present. Sadly some Android libraries or suitable replacements where available are absolutely necessary to run a phone nowadays. Both hardware and software producers are so focused on Android that drivers / binaries for alternative systems are not offered. That also underlines that this is indeed a niche product. Disclaimer: I ordered one of the phones.
I was always wondering the same and found the related choices always somewhat inconsistent. My own anecdotal experience was that Ubuntu was always to buggy to actually use it in production (every time I tried, I ended up removing it after a few days only due to driver road blocks and os-related bugs). Moreover, using Debian, as many said before, is a question of feature stability and extensive testing. That’s great for server deployment but not so great for consumer electronics. Thus trying to base a distro on Debian while trying to fulfill the expectation of receiving recent developments in the software seems counterintuitive to me.
Disclaimer: I’ve made a lot of very good experiences with Arch and derived distros and use them in production for more than a decade. Given my not so good experience with Ubuntu I’m certainty on the biased side here.