The rubbish bag can be pulled around the edge when the flap is closed. Then the bag opening is just a little taut so that the bag does not slip off.
If the edge wasn't hinged, you wouldn't be able to pull out the full bag.
Because the Müllex would be as small as the edge everywhere, the bags would not be properly full, i.e. you would pull out the supposedly full bag, but with a little shaking it would only be two-thirds full.
So the flap is actually a great optimisation.