Chad's videos aren't meant as a substitute for reviewing the sciences, practice questions, or practice exams.
The primary purpose of those videos is to give students an accelerated (most of it should be somewhat familiar) understanding of concepts in a short time, so they can strengthen the understanding of those concepts via review books, questions etc.
You should focus on understanding the concepts Chad presents, because that will allow you to understand what he didn't get a chance to cover.
In terms of whether what he didnt cover, and if is heavily tested or not, it makes most sense to cover your bases since there's no way of telling what version of the DAT your going to get.
But in a sense, yes, Chad covers most of what you'll see.
sorry for the long response lol, but I talked to Chad about how to best use the videos so I thought i'd share. gl.