@wut: Writers are not that meticulous. They write things in whatever manner sounds the best and don't always portray things correctly, I'm not saying that is what happened here, I'm saying you're going over this with a fine-toothed comb when that's not required.
As for what we've seen elsewhere from the custodes, much of that can also be chalked up to the fact that Astartes are simply far superior to any Orks the Custodes may have been fighting at the time. Hell one Astartes can rip through legions of Orks, and a small band of them can take on immense numbers with zero casualties. (See: Helsreach for Grimaldus and his Black Templars accomplishing just that). Orks are mostly fodder, they exist to be a faceless vessel for murder without mercy and most of the time even humans can kill Orks fairly easily. The Guard can fend off Orks all on its own, it's not like they're anything special.