Sure, but we draw distinctions like that all the time. There's generally legal differences between "E-Bikes" and "Motorcycles", despite the fact that they're nominally pretty similar (two wheeled, self-powered transportation). We draw distinctions between "phone lines" and "power lines", despite the fact that both carry electric current. We draw distinctions between "bread" and "alcohol", despite the fact that both are made the same way.
I'd argue that while there isn't a hard line in the sand, we more or less define "computer" as something that's "general purpose". I don't consider my oven a "computer", I don't consider my digital COVID test a computer, I don't consider my key fob a computer. I do consider my Macbook a computer, because I do a lot of dissimilar things with it; I write documents, I watch videos, I listen to music, I play games, I log into servers, I VoIP chat with friends, I edit video, etc. I don't think anyone disputes that a Macbook is a "computer"; if nothing else all of that applies to Linux and Windows as well.
You know what else it applies to? An iPhone. I can do all those things with an iPhone. I really can't do any of those things (besides play games) on a GameBoy.
Of course, admittedly I'm kind of moving the goalpost, because of course the line of "general purpose" is kind of arbitrary; the Gameboy did have a camera, the Gameboy Advance had a TV Tuner and MP3 player, so you're absolutely right that it would require some kind of bureaucratic overhead to define what "general purpose" even means, and moreover the second that they have a definition the companies will use that as a guide to narrowly skirt it and therefore avoid regulation.
I don't know the solution, but I do know that it feels a bit dirty for Apple to feel entitled to so much money when they're not even the ones distributing the apps at that point. People gave so much shit to Unity for their idiotic "install fee", but people have become bizarrely defensive of Apple for doing basically the same thing.