I think they just mean they should have control over the modem. They are all locked down and proprietary with known backdoors throughout history, effectively bypassing any OS level security.

A lot of phone modems ship with their own SoC (processor) running its own OS. It's much smaller and slower than the main phone SoC but, depending on its implementation, it can have full access to all of your main processor's memory through DMA.

I just saw discussions like this

So no, there are more parts to your phone. You forgot the whole part you, the user, solely interact with.