No need for empirical proof, I
did the analysis here.
The difference is: the early part of boot is Qualcomm code using Qualcomm security. These are the "pbl", "sbl/edl" and "aboot/fastboot" programs (and also "modem", "tz" and other bits). These were the parts that I was looking at in the link above.
When "aboot" completes, it hands over to the late part of boot, which is Android code using Google security. These are the "boot.img/Linux kernel" programs, "recovery", "system", "vendor", "data", etc. They use a different security model. That's what this root method targets. You are correct when you say "Maybe we are just so lucky that boot.img is not checked as rigorously".
It does imply that you can mix the PVG100 Qualcomm partitions for "early boot" with the PVG100E Android partitions for "late boot" and vice-versa. But someone with motivation needs to test this... (No, you can't unlock cellular bands this way; the "modem" partition is from Qualcomm and must match your hardware.).