If the chips are tightly integrated propriatary black boxes like on most hw then from my POV its _physcially_ possible for them to read anything regardless of what the designers/industry say because I do not trust them.
You trust your sources that say "..simply false that the cellular modem can access arbitrary data in RAM". I don't. Even if you claim to have personally designed, fabbed and shipped that silicon I still have no practical reason to trust.