In this case, that calculator is a red herring. The fact that it mentions AC and DC is not really relevant and is rather misleading. For a switch, you have to look at voltage rating, current rating, and whether it is AC or DC rated. Switches will often have both an AC and a DC rating and you have to look at the appropriate one. If a switch is designed for 12V DC, then pushing it to mains voltage means that you are trusting levels of insulation and safety for which it is not designed. It might be OK, but personally I wouldn't trust it. The contacts might handle the voltage and current (there is no magic threshold that says 12V is ok but 13V is not) but the casing and materials are not rated for mains voltage. Get a low-voltage circuit wrong and you might blow a fuse. Get mains wrong and it can kill. The toggle switch is clearly designed for mains voltage so is the safe choice.