The chance of getting no Raging Revenant stones in 2200 chests is (1 - 0.0475*0.09)^2200, which is about a hundredth of a percent. So either you're right that he can't be found in silver chests, or you're extraordinarily unlucky, or the odds are weighted. For example, if you get him only 1 in 100 times instead of 1 in 21, there's a 14% chance of not seeing him in 2200 chests. If the odds are 1/1000, there's an 82% chance of not seeing him in 2200 chests.
Given how often I see certain heroes (centaur, healer, megataur), I think it's likely that the odds are less than 1/21 of this guy.