So the official translation came out, which makes it clear that Zoro was indeed referring to Mihawk in this comment:
Zoro is implying that Mihawk is even less human than a killer robot. People have been speculating that Mihawk is less human because he might be different race. However, this doesn’t make any sense. Remember that S-hawk is made with Mihawk’s DNA, so that would mean that S-hawk would also have the genes of that race. Any “inhuman” trait that Mihawk may have in terms of genetics, S-hawk would also have, therefore that can’t be the factor that makes Mihawk less human.
The only factor Mihawk has that S-hawk doesn’t, is his life experience. S-hawk hasn’t lived the life that Mihawk has. I believe that Mihawk has been moulded in to a “monster” due to experiencing hardships and betrayal. We know that Mihawk has trouble trusting others, so there must be an event(s) that triggered him to feel this way.
People have been saying that Zoro made that comment just because Mihawk is stronger than S-hawk. While strength is certainly part of it, that can’t be the only reason. You wouldn’t call Roger or Whitebeard less human than a robot just because they are stronger, it definitely has to do with Mihawk’s personality as well. The experiences that Mihawk has faced has made him less human, which in-turn has made him stronger as well. I think Oda may be cooking something big with Mihawk’s backstory.