Nobodies have done the same.
They have battled Heartless multiple times, specially in HB/RG and hearts have come out of the Heartless.
you misunderstood...he said that merely attacking a human being meant that the heart will be removed simply because the scene where heartless attacked a normal person suddenly turned into a heartless. but that was only because a heartless was a creature who was able to do that. but he said unbirth could do the same jsut by attacking. but unbirths don't have hearts, so therefore cannot possibly simply attack a normal person and suddenly turn into an unbirth. and where the theory goes along, the heart extraction is still not in play even if an unbirth attacks a person and creates another unbirth.
i used the example of nobodies attacking humans and not turning them into heartless