As far as I know... the movement bonus applies to the slowest unit, which is the pikeman... plus, moving through territory that isn't yours slows down the hero...
I think that's how it works... if I'm wrong, please correct me...
The point is that the snake needs to move ONE unit of warriors across the swampy terrain. (see the description of VDV). The nomad cannot be dismissed because he helps to get through the desert (this time with the entire army). Therefore, the pikemen are dismissed. ewgeniy, is that what you meant?