Yes, but can he act without moving his upper lip?
IIRC that books takes place fairly early on in the Empire; very shortly after RotS, and yes at that time Tarkin was the only Grand Moff because he was the first Grand Moff. Others were appointed later.I was thinking of the Tarkin novel, where Palpatine appoints him Grand Moff at the end, with the implication that this was a unique circumstance and that the title wasn't one that existed until now. I could be misremembering, it has been five years since I read it.