On single-sided discs, you should notice that there is a single reflective layer between two 0.6mm polycarbonate slices. Your label is on the top slice, and the label is separated from the reflective layer by 0.6mm of plastic. The data is burned into the layer underneath the reflector, so it's even further away from your Sharpie label
On a CD, the label was directly on top of the reflective layer, separated only by a very thin layer of lacquer, which would sometimes allow seepage and corruption. That's a totally different scenario.