Pretty sure the stuff he's talking about is Calcium bentonite( I personally wouldn't use it.)
If so this makes sense being a seedling does better the more available Calcium it has as it can pick up more nutrients from the get-go.
Try a test..
Try throwing a 1/2 cup of gypsum(Calcium Sulfate) in per Cubic foot as well as your ostyer shell flour for a portion of your seedling soil. Put it up against the soil that doesn't have gypsum and you'll see a huge difference. It doesn't take much to start a seed, but when you have the right stuff in right proportion you'll see a different plant and root system.