Well there you go, if it says it in the rules then it is what it is. My apologies I was going off simple logic and as I don't own the Deamons Codex I don't know the specifics.
It's in the FNP description, not the daemons codex. Though, for the record, there is nothing in the daemons codex that overrides the standard, 5th edition printing of FNP. So, we gotta go by the book.
What you say BAZ, I agree with. To expand on that line of thinking a bit, it's not just the toughness that we gotta look at, but also the power of the hit they took. Things that would take a man's head off, I can see a FNP roll as being, "okay." Well, so as long as I've had a few beers, stand about 15 feet away, and squint, but I can suspend my notion of disbelief that far when it comes to GW.
Now, when we look at something that will not only take a Marine's head off, but only leave a set of smoldering boots, if GW were to allow that FNP, I'd probably have a harder time.
As a daemons player, and as a guy who occasionally puts plague marines on the table, I will admit that what GW did to FNP in 5th works. "Nerfed," I wouldn't say. "Fixed," eh, pretty close.