Asking MS about more CUs is never going to get you a straight or completely honest answer. Their system was designed around 12, they have no interest in telling anyone that more than 12 is a good thing, then people will make the obvious comparison to 18. Their narrative is the Goldilocks story. They chose everything just right, anything else would "unbalance" the system. 12 probably is a magic number for their system. Of course you can design system which perform great with more or less, hence AMD has a whole line of cards from 8 to 30 CUs.
Starting off your thought process with that statement immediately red flags it. You assume that a) they are lying or B) what they are saying isn't true. You have an assumption that numerically larger is better. But at looking at a system holistically that may not be true. They made a tradeoff and that tradeoff limited them in some areas but benefits them in others and maybe be better at producing a systemic benefit.