No, I've been putting all kinds of weight on that thing. I'm not worried about the tanks collapsing. I could maybe see a concern if the tanks were empty, but I think you're fine as long as the tanks are full of water. I'm 200 lbs and I stand on them empty and full. Obviously, Malibu has to put that warning on there to cover themselves, but they are pretty beefy tanks.
That being said, I'm certainly not condoning that anybody flagrantly disregard their warning stickers!