Kaizen didn't feel the need to tell Mirthor that he wasn't actually a Summoner, because it didn't make sense for him to tell him that.
Should Kaizen tell that he was not a Summoner, the facts of him being completely unarmed and with a Hellround Summoner at his feet would become much more surprising. Besides, Kaizen didn't completely trust Mirthor and his companions yet, so he thought it would be better to at least keep that information advantage.
Nevertheless, there was an information advantage that Psyker had and he wished he didn't keep it to himself, because while it was still an advantage, that information might make this quintet of adventurers change their minds about exploring this dungeon and thus returning to the surface.
Looking over his shoulder at the three runners behind him, Kaizen stated:
"Before we start exploring, I have to reveal something to you, something about what I saw the first time I came here."
