I've never heard anyone deny that people can be converted before death in my life.
Nobody denies this, so it's a strawman, and the reason it's used is to very subtly undermine EENS. "It's possible that this Prot was saved." But when you push on it, they can retreat to, "Well, we don't know whether he was converted ... unbeknownst to us." It's used as a fall-back position in case someone is called out on it.
Question is whether a Prot-as-Prot, who dies as a Prot, can be saved. Answer is no, not unless they convert first. BTW, the case of the Prot may or may not have anything to do with BoD as not a few Prots are validly baptized.
Then of course there's "Anonymous Catholicism" thinking where the Prot could actually be a Catholic without even knowing it himself.