That's hard to believe. Even Quectel (random Chinese IoT modem manufacturer, which is also using Qualcomm modem SoCs) gets to have Qualcomm modem source code. I know, because they ship it modified with their own AT commands implemented and other changes. If random chinese company can ship modified Qualcomm modem firmware, Apple surely can, too. :D

Massive chunks (millions of lines) of Qualcomm modem firmware (the part running on Hexagon DSP cores) are even leaked on github for anyone to see.

Apple is bound to have uptodate and probably even completely source available Qualcomm firmware at its engineers' fingertips. And they have more leverage than random Chinese IoT manufacturer, to request ability to modify it as they see fit. And they'll certainly have at least the parts that are relevant for the control you're talking about.

The decision most likely comes down to politics (any help optimizing qualcomm modems directly benefits everyone using them, and that's a lot of android phones out there), and not these kinds of technical issues.