This suggests someone may be able to install MacOS on an iPhone with some modification.

It's not the first Mac that has an iPhone/iPad chip. https://en.wikipedia.org/wiki/Developer_Transition_Kit

And yes, absolutely. All you need is a bootchain exploit. However unlike in the old jailbreaking days when people found and publicized them for fun, these days they are worth millions. Apple will pay you $500k for sandbox escape into the kernel. If you nail the bootchain, it'll be in the millions. From Apple. And god knows how much such a thing would go for in the black market.