Hmm, the whole idea of the Deepwarden class seems to be to emphasize solidity in defense, so adding a (unlimited by Armor Dex to AC restrictions ) CON Modifier instead of the DEX bonus seems fair.
It's a supernatural (extraordinary by the rules) power after all - and if one's winning smile and smooth manners can save a paladin from immolation at the hands of a fireball or petrification by a medusa's gaze(CHA Bonus to Saves), I can certainly imagine someone being so tough, that he actually shrugs off blows like water, too.
As the class through its prequisites is aimed at dwarven or gnomish rangers, it will be used by lightly armoured dwarves in the first place - due to the light armour limit on combat tyle feats.
And the "stone warden" power lets the character replace DEX with CON for determining the AC bonus. As armour restrict physical movement, an thereby limits the amount of mobility which can be employed while wearing armour, there is no actual rule that limits any sort of bonus to AC derived from CON. Hence, the power as written "floats".
Although balance-wise, it makes the class rather front loaded in a way.