Ethics exists within the "mind" of any agent which distinguishes "good" behaviour from "bad", or "right" behaviour from "wrong". In absence of all such agents, ethics is also absent.Well, no, not quite. Raolduke is saying that because morality is a human concept, it doesn't exist without humans. The justification was that no human concept actually exists unless we create it. But that clearly is not valid at face value, since gravity still exists regardless of whether or not we are here to theorize it. Applicability is a separate issue, but you're just as wrong about that as Raolduke is about the existence of concepts outside the human mind: The concept of ethics can and is often applied to animals and if we weren't here, the animals would still behave in roughly the same way.
So we'll need a better justification for why morality can't exist outside the human mind... Again, gravity. Humans are fallible and Newton's theory of gravity was flawed. Does that mean gravity doesn't exist outside the human mind? Of course not: just because we don't know the laws of physics completely, doesn't mean gravity doesn't exist.
So fallibility is also not a valid reason why morality/ethics isn't "real".