Waterproof inner pockets
The video shows that up to 78 kg of weight was exerted on the inflated waterproof inner bag without losing air or stability. It would have been possible to do more, but we ran out of hands.
Open the zip of the waterproof inner bag slightly so that you can fill it with air and then close the bag.
Test whether the air that you blew into the waterproof inner pocket escapes through pressure build-up.
If no air escapes from this waterproof inner bag, this is an indicator that it is waterproof. Since air is less dense than water, it is easy to come to the following conclusion: if no air escapes from the waterproof inner pocket, no water can get in either.