Recoil essentially comes from two sources : The momentum of the actual bullet exiting , and the gasses escaping .
A Suppressor slows down the exit velocity of the gasses , and spreads the force out over a longer time period , thus reducing the spike .
And heck, shoot a bolt gun in particular and pull the bolt after a shot or two. You can still see the gases swirling around in the barrel and can. It pretty significantly reduces the amount of hot gases exiting, not just spreading it out over a few extra milliseconds.