I'm a little familiar with that stand; I have the 24-gallon set up.
So I would think that the portion I marked in yellow is the primary downward load bearing portion. That said, a 40-gallon tank (water, sand, and rock) is VERY heavy, and even if the horizontal top (flat plane) becomes compromised, your glass tank may come out of level, and stress the tank. Only you have the eyes-on-the-scene, so it's a bit hard to say from that photo whether you are still good for years, or you're on borrowed time. Lastly, the fact that the outer skin is damaged, exposing essentially particle board, means moisture (spills) could cause a future problem. I hope my 2-cents helps.