NaOH is harmful to the skin in it's original state, before it has reacted with the fats.

Soap is not soap without the NaOH, so one can claim it is chemical free. The sodium hydroxide, which is derived directly from table salt, is what turns vegetable and animal fats from hydrophobic molecules into water-loving.

You can't wash with just vegetable ois because oil and water won't mix. The NaOH chemically combines with the organic molecules of the fats and places a "OH", hydroxide ion on the long chain fat molecule. This hydroxide ion then makes the fat molecule able to mix with both fats and water molecules.

So without the energy input from the NaOH molecule to strip a hydrogen from the fat molecule and replace it with a water loving -OH ion, soap wouldn't be soap, it would just be oil. The OH ion allows the soap to dissolve grease and oil from clothes and skin with the organic "fat end" of the new molecule and the other OH end, allows water to rinse away the dissolved grease and dirt.

Once the NaOH reacts with the vegtable oils, it is no longer NaOH. It is a chemical reaction just as gasoline is when it burns in your car and the byproducts are water vapor and CO2. Once that reaction occurs, the gasoline no longer is.

Same with NaOH. Once it reacts with the oils, it gives up it's energy and the NaOH breaks down into sodium surfactants. If there are any leftover unreacted NaOH molecules they can be rinsed out or reacted with a little citric acid to neutralize it and turn it back to salt.

So to answer your question, sodium hydroxide HAS to be used to make the corn oil and such into a soap. As long as you don't add any additional components after becoming a soap, his claim to be chemical free is true.