Sure, they could be, with a lot of effort. But they are neither marketed nor capable of general compute today. They don't have the toolset or ecosystem to make general purpose apps. Nor do they actively encourage people to make general apps.
It's simply that Sony won't approve non-gaming or non-entertainment apps. And if Apple were to follow the same path and block productivity apps then it would somehow stop being a general computing device ?
Why should someone else get to dictate which of my devices is a 'general computing device' and what that means for me and my use-case?
What is your definition of "general" that makes one of them general, and the other one non-general?
To me the only reason an iPhone seems to be "general purpose" is because the "number of developers has reached some critical mass and wants it to be". Which seems like putting the cart before the horse.
Sanity Test: Should my microwave be a "general purpose" device because it is built with a Raspberry Pi and a touch screen?
All have an App Store and are fully capable of facilitating payments, running Microsoft Office, using SaaS apps, doing "work".
Users are prevented from doing so by artificial restrictions on their App Stores which could be lifted at any time by a simple business decision. Apple allows more freedom to developers than these platforms yet somehow should be treated differently. Seems arbitrary and capricious.