For mathematics, all of the material is broken down into the very basic components, so it's easy for a video to have a set of prerequisites which any viewer should understand before watching (with a set of links to the videos corresponding to those prerequisites).
For a subject like history, the material can't easily be broken down. As the article points out, a statement like "as you can imagine, Japan did not produce a lot of its own oil" may be common sense to someone of high-school age, but an 8-year old may have no understanding of how oil is produced or where it comes for and the statement may make no sense. This doesn't render KA as dangerous or useless, just points out a problem which needs to be solved and which they are certainly aware of :).