Re: "Pixel also isn't a dumb terminal, is it?"
The issue is not the hardware. It's the software. A "dumb terminal" is something that's designed for remote processing and storage, and only display tasks are handled locally.
Apple iOS is designed to run and store data locally. ChomeOS is not, and most of its local features have been added later when it was clear the original design wasn't much appreciated. Still you wonder why Google can't use Android on both, like Apple does with iOS.
I agree with the first paragraph but somewhat disagree with the second.
iOS does store data locally, but not in a filesystem that's visible to the user as such in a typical fiile manager.