I looked over the factory fuel system and I think I understand what is happening here: even through, stock FPR is meant to work off vacuum, this type of FPR is capable of using boost to actually increase the fuel pressure! It isn't necessary designed to do this, but it can do an OK, be it not very reliable, job at it.

I guess being at only 5-6 psi, keeps it "safe enough".

This is all coming together... I might have to start a Rear Mounted turbo thread