> just how Apple cannot block their developer tools from being able to transfer apps onto an iPhone.
If I recall correctly (I might be wrong, because this was 10+ years ago), but Apple did exactly this when the iPhone was first released. When the iPhone first came out, Apple released its XCode devtools for free, including an iOS emulator that you could use to test your iPhone app. But you had to pay a $99 USD per year "developer program" free in order to use the devtools to test the app on your physical device.
If Google is also blocking preventing you from loading your own software onto your own phone with adb unless you pay a free, then this would be a very important thing to call out explicitly.
You recall correctly, but that did end in 2015, when Apple ended the requirement that developers sign up for their paid developer program to be able to develop and test iPhone apps. I've written about that elsewhere: https://appfair.org/blog/gpl-and-the-app-stores#fn:3
The adb workaround for Android is essentially on par with being able to use Xcode's tooling to install apps on an iPhone: technically possible without paying a fee, but enough friction that no one would seriously consider as an alternative solution for publishing their apps to a general audience.
> The adb workaround for Android is essentially on par with being able to use Xcode's tooling to install apps on an iPhone
The Apple situation is still significantly worse than ADB, because (at least without a paid-for developer account) AFAIK you're limited to a certain number of in-development app that you can install simultaneously and you definitely need to reinstall them every few days. ADB currently has no such restrictions.
Apple has actually increased the friction since: you now have to enable a scare-screened developer mode, reboot your device, install the app, get an error that the app is untrusted, then go to the part of Settings used for corporate management profiles to enable your own developer profile, and only THEN will the app actually launch and run.