I was under the impression that Honsou was not around during the HH, at least not as an IW
At the end of Angel Exterminatus it shows Fabius Bile with a geneseed tube, the only viable one saved from a fire. Only Hon... Sou... can still be read on it. An iron warriors character in the story is Honourable Soulaka. So pretty easy to guess whom his sire is.