Only on-disk, non-addressable controller proms are "read" by the software in the proms.
The boot prom has to boot stuff or the product can't be sold, and in this case is used to boot a program that runs on the hardware that continuously reads the prom. That HW can verify it, and all the other proms which are reachable from the CPU, including all sorts of stuff plugged into the various busses. That includes some disks, the ones we were worried about viruses wiping.
For some specific disks, you may have to pull the drive and clamp directly to the prom's pins.Those are the ones a spy would want to subvert.