Mobile OSes are special purpose, constrained to their phone + apps, developed by on external devices.
Being able to develop software for a device on that device also isn't the definition of a general purpose computer.
A general purpose computer is one that can be easily made to run different programs. Again, it's a differentiation between devices like microwaves and ovens that are technically computers, but aren't used like computers by the general public.
I guess I will prepare that accounting document with a Bluetooth keyboard paired with my fridge.