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.
