Harkam raised the Colossus right from when she was a young child so he knows a lot about her. He is like a father to her and his word should suffice. Unfortunately, no one is buying that a transcendent is imagining things and making stuff up. Not when Harkam himself had expressed the same thing just a year ago.
He continued to talk when he noticed that no one is buying his excuse. "How about that Helios boy? He has a good head and a smart mouth. Look at how he refused to give up the ants we captured so easily. That's a divine lord in the making."
Harkam is willing to resort to something he would never admit in any other condition to divert attention from him. He complimented Helios openly.