No I think he's wondering why Revenant is doing true damage, if I'm not mistaken. I have no idea how that happens sorry, can't help with that. Pretty sure he's not supposed to deal true damage. It's probably too late but if you checked his skills you could see if the damage amount was a multiple of his shroud skill or whether it's come from elsewhere (his basic is about all else it could be unless I'm missing something). But either way it still seems like a bug to me.
Edit: his shroud skill hits all foes not just horse, so I guess it must come from his basic or some weird glitch.