That's quite false. It makes no mention of throat-slittings.
The specific text you get when you are seriously wounded by an infiltrator (ie, the worst it can be), is
Even if he catches you sleeping, you always wake up in time to grab a dagger and at least try to defend yourself. Your wounds are body-wounds at worst, not neck-wounds.
And these days, it can even turn out to be not that bad—his blade missed the vital organs, or maybe the blood on your shirt was from a smaller scratch and the major wounds were only in your thigh and shoulder—and you only get a regular wound.
You manage to sneak into his sleeping quarters unnoticed.
Silently, you sneak up on your target, and surprise him.
You quickly put him down for good, seriously wounding him before you make your escape.
How does that implies that he even woke up?
Since an infil can't kill a noble why can he atleast be able to kill a Hero or an advy? People choosing both of these classes know that their characters may die and should be prepared for that. You just add an extra risk.
