That's not really a fair statement. I have kessil's only on my tank, and i've had great growth out of all my corals. No supplement lighting at all. My Jedi Mind Trick Monti and my Red Cap Monti and my Reverse Superman Monti have tripled in size over the past few months.
This is my Reverse Superman Monti when I first got it.
This is an older picture taken about a month ago
I'll try to snap a picture tonight when I'm done with work. But all my SPS corals are doing great under just Kessil lights only.