if Dante can survive getting shot in the head and his brain presumably turning to mist, demonic regeneration probably could regrow limbs if it kicks into high gear.
That being said, given Nero's arm had a) been gone for over a month and had healed enough to use The Most Souped Up prosthetics imagineable and b) his arm can conveniently turn into energy even after it grows back, I'm saying it's basically just a magical. Demonic. Whatever. Prosthesis.
It's a very good prosthetic!
...Unfortunately it's hard to take full advantage of it in a way that doesn't lead to awkward questions about how his missing arm randomly appears sometimes, so. Special Occasions only















