Here are some pictures of the new GM seal for clarification. As you can see, there are two sealing surfaces. For reference, the yellow side of the cover goes towards the outside of the engine and the silver is the inside. There are two sealing surfaces- one is a ribbed rounded surface and the other is a lip seal. Neither look as though they would seal much pressure into the engine when installed as per the manual.
From the outside looking in:
The lip seal:
The rounded ribbed seal:
From the engine side looking out:
The rounded seal and lip seal both taper towards the outside of the engine.
It doesn't look as though this would hold much pressure in. You can see that if the seal were installed backwards that both surfaces would act to keep pressure inside the engine. Pressure would expand the lip seal towards the crank surface, tightening it up.
One disadvantage with installing it backwards would be that it would not seal against debris getting in very well. Not good for an off-road rig.