A consistent style of illustrations is part of the guidelines for most online brands. All the illustrations need to share a common language across the whole product.
* Atlassian has a style-guide
* OK Cupid has one (they just changed it and had to redo all their illustrations)
* Trello has one
* Tunnelbear has one
Having a traditional artist involved in these illustrations rather than a designer means that the 'hit by a bus' factor is much larger. It is much easier to imitate a vector art style than it is physical technique.
Furthermore, these illustrations have to share the same guidelines as the app itself, which is limited by CSS rules. Natural textures are hard to do in CSS and wasteful, wheras flat colour, borders, and gradients are all up for grabs.
On a similar note, one could complain that the New York Times have terrible consistency with their illustrations, and the differing illustrations fail to portray a cohesive brand image.
So ultimately the author is saying that marketing != art... No shit...