I'd chalk that up to the DM's choice.
But that's just because I believe they refer to the bonus hit dice as hit dice instead of as simply 2d10 temporary hit points because then, as per the rules, it wouldn't stack with the temporary hit points gained from Con mentioned in the following line. I mean, the rest of the bonuses granted by the ability aren't HD dependent things after all except for the last bit about the additional dice counting towards total HD in regards to spells that check HD.