If they under filled the tank, it would be completely obvious to the diver. All diving gear has a simple pressure gauge that displays the air left in the tank. Divers check the air gauge frequently while diving. Even if the tanks were under filled, it wouldn't make the dive unsafe -- Divers are trained to end the dive when the tank gets below 25% remaining.
There is no reason or benefit to under filling tanks anyway. Most divers just use compressed air which costs nothing, so the company wouldn't be saving any money by under filling tanks.
The whole idea that the tanks might have been under filled is the dumbest thing I've ever read on the internet.......