Where did he read that? How about in the article?

The original article states: "One of Python’s most visible users, Dropbox — who also employs Python’s creator Guido van Rossum — recently announced it has migrated major parts of its back-end infrastructure from Python to Go" and refers to [1].

[1]: https://tech.dropbox.com/2014/07/open-sourcing-our-go-librar...