It’s worth a watch, but for the theory itself essentially the effort/resources required to colonize planets that are not earth-like is unsustainable. A lack of a self-restoring cycle for resources and general ecological needs would inevitably lead to one of a variety of possible collapses. This would lead most intelligent life to avoid extensive expansion due to the heavy cost. Issues brought up with the theory in the video was stuff like tech advancement improving the practicality of life in free space as well as the possibility of differing biological needs (i.e. if a species went more mechanical or used AI/machines they could bypass the need to terraform or transport basic biological needs).

A lot more and better explained in the video, but that’s the gist

Take the time to watch, Arthur is worth every second