The P90 was designed for vehicle crews, operators of crew-served weapons, and support personnel who needed something less than a service rifle but more than a handgun. Body armor was not considered other than being able to shoot holes in it.
Very good point.
This is a very good explanation of your thought and knowledge of the weapon system.
Thanks for expanding why you didn't agree with what I posted.
Considering your info is correct, that would mean the shortness of the platform is a bonus to service users who would have to use the platform while wearing a thick service outer carrier and/or other equipment without overextending the hand grips too far forward to hurt ergonomics.
Tying this back to the OP's question, I'm not sure why the edges of the stock backplate would slip off the shoulder. But for taller people, the shortness of the rifle could cause them to hold it forward some such that a proper shoulder to backplate weld is compromised such that it could slip out of position.
In this case, I can see how a recoil pad can improve the shoulder to backplate weld.
And I agree with you: he and his wife should go out and shoot it to make sure they wouldn't really need to take this measure.