I don't see a problem.
Loads of people keep bettas in unheated un filtered 1/2 gallons. there's no cries of damnation about the sale of those tanks. And the larger ones are even better.
1/2 gallon of water will weigh around what? 2 kilos? a wall could take that no problem. So long as the screw goes a fair amount in. 8 kilos for the largest is not that bad.
As for them saying what you can put in it, almost all retailers (especially those aiming for large sales) will overstate what can go in a tank.
All in all, I think it's quite a novel idea. similar to those 6 inch deep tanks you see to appear like hung picture.