They have been since the beginning.
That’s literally why I got an OG Motorola Droid. I said to myself “I can have a full computer in my pocket!”

The whole point of the article is that the new MacBooks are running on iPhone hardware. And that therefore there’s no reason for you not being able to install MacOS on your iPhone. Even your old droid was locked down and you were not able to install a real OS which would have given you the freedom to run what you want without restrictions

The only difference is UI. I operate my phone in my hand with my thumb, not on my lap or on a desk from a keyboard.

And the hardware. Your phone requires much harder power optimization in order to have a usable battery life. Same for size and heat dissipation. Also politics. Public cellular is tied to identity. It is structurally hostile to user-controlled, fully open, deeply optimized devices because the radio stack is certification-heavy, operator-governed, and privacy-hostile.

For the first point, that sort of engineering is more necessary for a smaller form factor, but can absolutely be done for other computer hardware. Applying these lessons to laptops yields some amazing laptopsz for example.

As for the second point, that’s just one component in the system. You can attach a cellular modem to a PC. They just don’t tend to be built in directly.