I'm not sure you understand what a non-newtonian fluid is. A toothpick would float on jello, but if you stick it down into the jello, its buoyancy doesn't factor in. Quicksand compresses and becomes solid under force, and if you try to walk out, you're more likely to pull yourself deeper. The force of you floating is weaker than the force of compressed quicksand keeping you down.

And I didn't say that anyone drowns, I said it gets hard to breathe if you manage to sink up to your chest. You're right about the dehydration and fatigue, but it is due to panic. If you can float, then you could just lie on your back and roll your way to the edge. But once you're in deep, it would take a tow truck to drag your legs out against the force of the gelled quicksand.