That books looks anything but mini. Plus I don't know how one can judge it's thickness, as we see only 2 dimensions of it.I don't see them putting DA on the cover of the Main Rulebook and it looks a little thin. So maybe the Limited edition Boxed set has a hardback mini rulebook inside.
If you look at Fantasy and the past 2 Hardback Rulebooks they moved away from any specific army on the cover. They only had symbols as thats more generic.
It's somewhat of a combo breaker, but space marines on the cover of the rulebook is not unheard of. Maybe it's GWs way of saying that the pendulum swings back to more immersive rules.