I wonder (and can not prove, but remember this concern at Qualcomm and Apple back around then) if the issue was forced by Cingular Wireless originally.
In particular, iOS devices were dependent on carrier cooperation, and Cingular may have required only vetted software builds to connect to their network.
And now with their custom silicon, no other system is officially supported and your "options" are basically hacks that can only use some parts of the hardware.
The platform is notoriously undocumented so it's not like if Apple is making it easy to install any other system. In fact, the best you can say is that they didn't outright block running other OSs but you can't really use the hardware properly that way (for example the fingerprint reader).
When it comes to iThings, they actually go out of their way to make sure you can't even install any version of the OS than what they chose (sometimes people run into cases where it's extremely annoying to be unable to downgrade the OS because the hardware has become way too slow to be useful).
So, when you say they helped significantly that makes me laugh, they used the fact that their hardware that was very close to the common PC could indeed boot other systems as a marketing tool but not much more.
You can, today, for example, install other OSes on Apple hardware, and they themselves have helped significantly to enable such until relatively recent architecture changes.
I, for example, run Fedora on a modern Macbook Air and it's super useful.
It's possible this reality deflects some of the antitrust attention.