He couldn't believe that the designers had ignored his opinion and installed it anyway.
Interesting. I've never read the novel, so I didn't know that Kirk had made that recommendation. I only know what's on screen- that Kirk didn't know about the change but Decker's knowledge of the ship saved it from crashing into the asteroid.
As an aside, I hope Timo sees this. I enjoy his rants about Kirk's leadership by arrogance approach from TMP.