Hello; The actual pictures do not seem to match the diagrams. I count seven internal supports (panneaux de contraplaque) in the diagram and only four in the picture. Perhaps they are doubled up?
The diagram also shows front and rear panels (portes donnant acces) that are not pictured. These panels should add strength in a lateral direction to prevent sideways torque. They may also add some vertical strength. Both types of strength depending on how they are fastened.
The material appears to be wood. It is hard to judge the thickness and also what type of panels. It would be interesting to know how the panels are made and how they are bonded. Some plywoods are quite strong.
If the tanks are to be filled as pictured, I would be a bit concerned based on the limited information. As pictured I would likely not use that stand without knowing more. I am a pessimist by nature and tend to want things stronger than necessary.
Good luck