Earlier quoted context omitted.
The phone being off or it being on airplane is no longer enough. It is known that complete operating systems run on every chip on that phone of which you don't have knowledge of or access to. To think a software security solution provided by an OS, a pretty high-level abstraction when considering hardware, of the ability to turn off the radio is insane in these days and ages. Furthermore with permanent batteries (or…
I'm curious as to where this backup battery thing came from? Surely the likes of ifixit would notice extra batteries. If you're going to assert that such a thing is still hidden, then frankly, your mindset is such that there is no lengths you can go to protect yourself from... whatever? I would think a phone with a removable battery would otherwise serve your concerns.
The sentence was about permanent batteries with an aside about how technically a remove-able battery phone could still have power somewhere with it removed.