In this instance, my instinct is to say no. Although the supposed duplicate makes reference to the incident in question (Frodo's stabbing with a "Mordor-knife") it certainly doesn't offer any conclusive, definitive proof regarding the identity of the person doing the stabbing, which is the whole point of the earlier question, "Who stabbed Frodo?"
Frankly, I'm uncertain why you feel that this particular quote offers enough proof of the identity of the stabber that you'd be willing to consider an acceptance on the basis of it being added to an answer.
"Escaping a wound that would have been as deadly to him as the Mordor-knife to Frodo (as was proved at the end), he withdrew and hid for a while"
Nowhere does it say who the Mordor knife belongs to, and hence this quote doesn't answer the question. If you could somehow show that there was only one Mordor-knife being used in the fight (or that the other Nazgul were wielding something other than Mordor-knives) then you might be able to construct a case that it's been answered by exclusion.