There's "wrong to not pay him" which is distinct from "he was entitled to be paid under terms of contract" (which itself would have multiple interpretations based on who was reading it, etc.).

They were uncontrovertibly wrong to not pay him, because they've lost 10-100x $10k in goodwill, recruiting capability, etc. If they really wanted to be strict and avoid establishing a precedent, they could have just given him an extra $10k bonus for severance vs. $10k for the referral amount; I doubt he cares what the memo line of the check says.