yeah this looks correct and it's a different world if you have direct access to the modem IC.
To clarify I was referring to the modules that the average person can get their hands on. If you rip open the module (or look at the FCC paperwork) you'll likely see a Qualcomm chipset underneath. It's just a level of abstraction really.
https://en.wikipedia.org/wiki/Qualcomm_MSM_Interface
I think non SoC high performance modems use USB.
Regarding older or less integrated modems that do use the 232 interface, this may be illuminating:
https://fahrplan.events.ccc.de/congress/2011/Fahrplan/attach...