The point of doing it this way is the following so much as I can gather:
1) Start the biological activity in the substrate. Any substrate will provide a multiplication site for beneficial bacteria. Clay based substrates will be superior to plain old rock substrates. However, the technique is beneficial regardless of substrate choice. The only possibility is that the apparent nutritional level of Aquasoil may benefit the plants in early growth. Either way it looks like a good strategy.
2) It builds a biomass thats well rooted and developed to effectively stabilize the tank when filled with water. Again, useful regardless of substrate.
Don't get caught Jonesing(
http://www.urbandictionary.com/define.php?term=jonesing) on the substrate choice or efficacy. Don't forget, plain old gravel works just fine.