07-08-2012, 01:22 AM
They do give you indication of that, but you'll only understand the reference once you've played through Scrapyard Defence. The mission description for Tango & Cash specifically says that it applies "the next time zombies invade" (thereby indicating that it doesn't trigger the invasion itself, and requires some other mission or player to do so).
I think the intent of this one (and its time limit) is to encourage other players to help out multiple people playing through Scrapyard Defence, since you can cover two or three attacks within the timelimit and you might not make your quota with only one.
But it could have been made more obvious.