Let's not fetch too far, this becomes a strawman argument.
They weren't compatible operating systems because applications compiled for one of them were unable to be executed on the other without heavy modifications.
At "underlying layers" the OS of a Tesla is compatible with that of a Nintendo Switch, and yet no one would say they have a compatible OS.
> It is like telling Samsung, Huawei or Xiomi aren't Android, because they use another GUI framework on top of AOSP.
No it's not, because they all use the same GUI framework as AOSP, hence they can run the same precompiled application.
"In terms of software, Satio uses the Symbian OS 9.4 operating system, which is created collaboratively under the stewardship of the Symbian Foundation as "Symbian^1".[4] It is Sony Ericsson's first non-UIQ Symbian device, after UIQ's development closed down earlier that year. It uses the PlayNow service, Sony Ericsson's mobile content platform, and is part of the company's new Entertainment Unlimited service.[5] In terms of connectivity, it is Wi-Fi-enabled and has a GPS chip for navigation and location-based services. It also supports full Flash for video playback.
They made a total of 2 devices with Symbian^1 (+1 refresh) before ultimately moving on to Android.
Symbian^1 was the attempt to harmonize the Symbian flavors into one open platform in order to compete, after Nokia, Sony Ericsson, Motorola and other contributed all IP into the Symbian Foundation.
Nokia called it "S60 5th Edition", for others it was "Symbian^1", and the first "common" Symbian OS.
So yes, I wasn't precise enough in the later comments, and you're correct.
(Sony) Ericsson used UIQ, a pen-based OS built on top of the core of Symbian foundation.
Nokia developed Series60, a key-based OS built on top of a Symbian core.
They were not compatible operating systems, and most of all Ericsson didn't license it from Nokia.