It would make sense, given the comparative power level of Zariel we're seeing here. She and Demogorgon have the same CR, but he's arguably the strongest Demon Prince while she's likely one of the weaker level-ruling Archdevils.
And it could easily be explained in gaming terms - the versions of the Demon Princes seen in OotA could just be avatars, or similarly weaker versions, as the result of the botched summoning in that adventure. The Demon Princes' true forms would likely be somewhat stronger.