The author's interpretation seems to be that the spirit of "child-like playfulness" was already the dominating design paradigm at Google, as characterized by Sa

The author's interpretation seems to be that the spirit of "child-like playfulness" was already the dominating design paradigm at Google, as characterized by Saville in 2010, and the re-design of the logo was done in this spirit. It's pretty plausible, given that this was the first time Google was changing its logo in ten years. Edit: FWIW, it's not designs that are just flat that the author considers child-like. It's a design vocabulary that uses flat designs, cartoonish characters (e.g. Octocat), addressing users in a child-like manner that's seen as childish. Flatness is just one element of this vocabulary.