Hm, I was always under the impression the Defiant only had 2 forward launchers... Maybe it means 2 Photon and 2 Quantum?
Anyway, it also states (describing what you just said):
"Nog modifies
unspecified torpedoes with photon torpedo casings in this episode. When they are fired the
effect is identical to the quantum torpedo."
I've underlined those two words, who says they're not Photon torpedos and the mods Nog makes merely gives them the appearance of Quantums? Or that the FX or props guys got confused or lazy?
If you take everything you see on screen as canon you'll be stuck in paradoxes, conundrums and hypocrisy.