Well, Dante is 18 in DMC3. He's about 30 in DMC4. Nero's, like, 18?
Plus, I find it more likely that Dante was referring to the presence of Vergil within Nero, hence Nero having the "blood of Sparda". I mean, the whole incident with his arm was never explained properly, but there was that cutscene when Nero awakens his DT and says that he heard a voice saying, "Power, give me more power!" which is one of Vergil's famous phrases, and then Nero said he gave into it, saying, "If I become a demon, so be it. Anything to protect her." Which, to me, seems like he was gonna become a demon because of some sort of Vergil encounter, but accepted it because the powers gained from it would give him the ability to protect Kyrie. That's my basic two cents.