The ratio of occurrence of hydrogen-ammonia planets and these super-dense water-oxygen worlds of theirs over the entire Galaxy--and remember that they have actually conducted a survey of significant sample volumes of the Galaxy which we, without interstellar travel, cannot do--is about 3 to 1.
They have used a large sample of volumes.
That would be all right if he could offer some other coherent way to think about these issues, or even if his critique were arresting or persuasive.