I'm not entirely sure the author himself understands the difference between data and metadata. It has nothing to do with importance; rather metadata is "data about data". The author seems to think it's got something to do with a distinction between system and user data, which isn't the case. It's an important logical abstraction to make, as "data" is a noun, whereas metadata can be thought of as a collection of adjectives.

Now if you want to start discussing why "data about data" should be considered part of the data itself, that's an argument we can have... but that doesn't appear to be the goal of this article (at least if it is, he doesn't make much of an argument on that level).