He only had a minute-thirty to explain the whole process so it makes sense that skipped a few details. If he believes the gas produced by fermentation can be burned it would be fun to watch him try and light it.
Every livestock farm on the planet produces methane. When not managed properly they become extremely aromatic. The basic concept behind the bio-digester is compost in a contained environment. If you have ever seen the big eternal flame that burns at your cities waste treatment plant you have seen this process in action. They are burning off the methane from the waste.
It's a great idea actually. Japan has been building methane capture systems into their landfills for decades. If you have ever played a round of golf in Japan you probably were walking around on one.