From post #1 of this thread:
Assuming you already have TWRP, take the attached zip file and flash it on TWRP thats all!
As you said, some users decide to turn off root (for reasons unknown to you). Even so, my impression is that TWRP continues to have root access to the device for as long as it is the device's recovery. This is a quick and dirty summary of the relevant parts of the link I provided.
The part that you quoted from is directly respondent to this question:
If I connect the phone to a PC then doesn't that give me access to the entire file system of the device? If so, then why can't I just overwrite the relevant parts of the FS with whatever ROM I want?
The reason he says that you need root to do that is because it's the access that the device's OS provides to the device that is attempting to read the device's filesystem.
Anyway, I acknowledge that this discussion is entirely academic at this point. I was wondering why the camera requires root. As it turns out, while your argument is that it does require root to install, it's clear that it doesn't require root at any point to use it. That's the answer.