My knowledge is third-hand as I haven't worked at Motorola for over 20 years (and back then I worked on "server" side, not device side), but I think you are correct. What I'm unclear on is how it differs from the chips (and associated blobs) sold for use in China, where the legal requirements differ, and how reliable the "signing" really is. I do know that some groups get special permission from Qualcomm to load other firmware and expose certain features to the user-land OS. My threat analysis is mostly theoretical and I can't really say that Qualcomms control is lacking or co-opted.