No, he can't. And you can't prove him wrong either.
I dont know about you, but where I come from we use facts when we argue and not speculations. But if go by your logic there is no point in aruging at all cause nothing can ever be proven according to you. But, lets look at what he wrote again, he claimed didnt say "I think" but claimed that if not lowering the kingdom size people will leave. People have been leaving the game for quite some time and it certainly havent been due to kingdom size. That suddenly it will be due to kingdoms being 25 and not 20 is not a plausible argument. Your above line is quite childish.

They don't make money because there aren't enough players to get enough from advertising to do more than cover the server costs.
And why isnt there enough players? ...