GPIRS is vulnerable, though, and given that something as new and shiny as a 787 can be confused by the GPS spoofing and decide that it’s 80 miles off track, a 737 is likely to be affected in a similar way.
Even if you ignore the vulnerability in the GPIRS setup, the FMC is also susceptible to spoofing. There’s a reason the Iranians plonked a DME just inside the border that broadcasts on the same frequency as the one over the hedge in Iraq.
I don’t think you can trust the FCOM here - Boeing swear blind that the 787 can’t be position-spoofed when I’ve had to intervene multiple times in Iraqi airspace to stop it turning towards Iran in LNAV.