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.

I don't think the cellular network is the problem at all - everything except SMS and PSTN calls works on wifi. The problem is the apps. Netflix only runs on a verified bona fide electrified six car Google- or Apple-approved device; so do most financial apps (EU law requires them to) and basically everything else where the app developers are trying to get money off you (which is most apps). Some apps will refuse to play ads on a non-genuine device and then refuse to function because you aren't watching ads. Play Store does its best to stop you installing its apps on a nongenuine device, but it has to support older devices without TPMs so it's not fully locked down yet. Even YouTube has some level of attestation.