The test is about whether such an evacuation is theoretically possible, not if it is realistic under any conceivable circumstance (half the plane gone, blocked

The test is about whether such an evacuation is theoretically possible, not if it is realistic under any conceivable circumstance (half the plane gone, blocked exits, plane resting at an incline, smoke and/or fire and/or water in the cabin...). If the happy-path evacuation isn't possible to begin with, e.g. for design reasons, the more complicated realistic ones will be negatively affected as well.