Are you talking about a "dumb" phone? That I would agree is a special purpose computer.
A smartphone is definitely a general purpose computer, though. The definition of a general purpose computer isn't "a computer that can do anything a typical PC can".
A proper definition would be something like "Any computer-based device that accepts different applications". It's a differentiator between things like microwaves, which are technically computers, and smartphones/laptops/PCs/tablets.
So how do I run Android Studio on my general purpose phone, without workarounds like connecting to a VM in the cloud, while looking at an usable screen?
Mobile OSes are special purpose, constrained to their phone + apps, developed by on external devices.
You don't run Android Studio on your phone. Being able to run Android Studio isn't the definition of a general purpose computer.
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.
A smartphone is definitely a general purpose computer, though. The definition of a general purpose computer isn't "a computer that can do anything a typical PC can".
A proper definition would be something like "Any computer-based device that accepts different applications". It's a differentiator between things like microwaves, which are technically computers, and smartphones/laptops/PCs/tablets.