That's not what I said! If the layer you have has a 'D' face, then I'd want one on top that didn't! You never want a void in a floor you are going to tile. The fact that it is strong overall has nothing to do with a point load over a void. A small deflection there can crack a tile. Natural stone wants two layers, but normally, one over dimensional lumber is fine for either (if it meets industry standards, that is).