There is no reason why Valve would have made the Dynamite Pack a secondary, especially considering when you look at the internal weapons for Demoman. The Stickybomb Launcher is described as the Pipebomb launcher. The Pipebomb launcher would've co-existed with the Grenade Launcher.
Looking at the code suggests that grenades worked like TFC. The MIRV is the Dynamite Pack. The MIRV is a grenade. The MIRV would have never been a secondary for Demoman, it just doesn't make sense. Even considering the fact that in Trailer 2, he holds the MIRV in his left hand, lowering his Pipebomb launcher.