You could, but if it's in the app store for the masses to do, most of them won't be, and that's sort of the point of the suggestion here in my mind. Easy self-hosting for the masses with the device they already have.
There are lots of old phones out there, and rather than getting binned it's nice to think of them being put to use. With the exception of the rather young, nearly everyone's had more than one phone in their lifetime, and even an old smartphone is more than powerful enough to run a simple webserver.
I agree with you, but I think your missing my point. If I go and put something like this in the Play/iOS stores, what percentage of users looking to run a small site do you suppose run it on the phone they use everyday vs running it on a wiped old device that they have plugged in in the corner? If you think most will take the proper precautions you have more faith in humanity than I do. Personally, I think even with the somewhat nerdy nature of the app >50% will be running it out of whatever is in their pocket, no matter what kind of warning banner I might put somewhere. And I'm nopeing out of handing that foot-gun to them.
If the app is sandboxed, then it's the same level of risk as browsing the web on your phone. Sandbox escapes are rare enough that the people who find them aren't deploying them against randos.
