Indeed, if this is true, the plot for this dlc is even more interesting than what I thought.
Definitely, would make things far more interesting.
I'd love to see some talking Falmer NPCs, at least if you're on the vampire side, who could tell us more about their motivations assuming they can still communicate. And maybe some battlecries while fighting them as the Dawnguard

. It was annoying that the main game made them a generic Goblinoid creature enemy rather than an anguished ancient elven race of former slaves. I'd imagine some might still be capable of speech (they seem to have some semblance of a culture down in the depths with armor, domesticated Charrus and huts), unless the vampires are controlling them completely and that's the only reason for their cooperation.