We should have the ability to run any code we want on hardware we own

This makes the point that the real battle we should be fighting is not for control of Android/iOS, but the ability to run other operating systems on phones. That would be great, but as the author acknowledges, building those alternatives is basically impossible. Even assuming that building a solid alternative is feasible, though, I don't think their point stands. Generally I'm not keen on legislatively forcing a developer to alter their software, but let's be real: Google and Apple have more power than most nations. I'm all for mandating that they change their code to be less user-hostile, for the same reason I prefer democracy to autocracy. Any party with power enough to impact millions of lives needs to be accountable to those it affects. I don't see the point of distinguishing between government and private corporation when that corporation is on the same scale of power and influence.

The primary problem is that we can't build a phone and run it on a cellular carrier network. This is where legislation is needed.

Apple and Google are still a problem, but they are a secondary problem.

You kind of can? The carrier network has no way to verify that your cellular modem is a real modem made by a real modem company, and not 3 SDRs in a trench coat standing on the top of each other.

The sheer technical difficulty is what makes this kind of thing impractical.

The network does validate that a SIM card is a real SIM card, but you can put a "real SIM card" in anything.