The Runefang is a magic weapon that autowounds and does not use any strength at any point in the process. Its chance to wound is not based on any strength value, nor is the armour save modifier. The Pendant Lord can make a save against any strength-based attack. To call a weapon that autowounds an S4 or S=wielder weapon is, I'm very sorry, most silly, and has no basis in the rules. You cannot autowound with an S4 attack, and if you do, it's not an S4 attack anymore.
The fact that the Sword of Hoeth explicitly mentions ASM supports my PoV, and the same goes for a Ghouls poison attacks. They need to use some value at some point because they do allow armour saves, and therefore it must be defined.
The Runefang has no such limitation. It is a different weapon from both the Sword and poison, neither of which can therefore set a precedent, and its rules do not mention a certain strength or ASM.