well, I'm not sure about the trash theory but that may be part of it. I'm thinking this pump can handle just about any amount of paper cups and plastic bottles you could throw at it, one at a time or all at once. The bigger trash won't make it through the bar screen. Trash is definitely a concern because this is a storm water pump.
It would prevent any vortexing from occuring above the pump inlet. The plan also shows significant other features designed to prevent vortexing or stagnation in the corners, along the bottom of the walls and below the pump intake.