[Marauder]

The original task was for the other 12 HM's not to find out that Mario and Lisa weren't a couple. Mario being with Steph was going to be a "cover story".

When they easily managed that, BB then amended the task to include that Mario and Steph had to go through with the fake wedding.

When they just about managed to go through that convincingly enough, with some people being obviously suspicious (who wouldn't be), BB then pushed the HM's towards the truth by telling them there was a secret couple, they then didn't have a chance.

BB didn't think this through. They didn't stipulate what exactly would constitute a pass or a failure. Prior to the "announcement", no one openly accused Mario and Lisa of being a couple, so they should have passed the task.