Trust me everything this guy builds in his videos WORKS and WORKS well.
I have recently just built a stand using his design and wow it is strong. If you subscribe to his youtube channel, you will not be disappointed, one of the best DIY sites for information.
There are many people who have built even bigger tanks and proven it to work. Plywood tanks are not new but bare in mind you only see benefits on cost (biggest being glass or acrylic) for the front panel above 220g. The covering stuff (Zavlar Liquid Rubber) is primarily used to cover roofs as a waterproof membrane so it is incredibly waterproof and extremely cheap and easy to use compared to fibreglass or epoxying the tank.