Same difference, they are just using a built on plenum/shroud on the Swifty and Black Ice radiators. If you look at the pic of the 3 radiators, you will notice that the sides are closed in and the mounts for the fan are raised off the surface of the fins. I guess you could say that a plenum is a built on shroud whereas a shroud is a bolt on plenum.

The shroud I use on my watercooled dually effectively acts the very same as the way the plenums on the Swifty and Black Ice radiators do in that it seals the area between the radiator and the fan for more effective airflow and mounts the fan away from the radiator fins for less turbulence, hense better airflow.