From the comments on that video I found this site with this succinct explanation:
The Chthonian Theory states that the Earth started its existence 4.6 billion years ago as a gas giant. The fusion ignition of the Sun stripped away the Earth's outer shells and thus it became a chthonian planet. Approximately 180 million years ago started its rapid expansion as a result of force balance between the cracked rigid mantle and previously highly compressed interior. The expansion led to a creation of the oceanic lithosphere and Earth's primordial heat loss. It is a competing theory to the theory of plate tectonics and thus e.g. rejects a process of subduction.
From the comments on that video I found this site with this succinct explanation: