Having the developer of Homebrew as an employee is only an advantage if you want to influence it's development, and if the developer is happy for his application to be influenced by his employer rather than the Homebrew community as a whole.
In fact, I would see Google hiring the leaders of open source projects in order to influence the direction the project takes as a problem. Sponsoring an open source project is brilliant, and Google are great at that, but buying influence over it is completely different and (potentially) an issue that would lead to people forking the project.
If interviews are really made to maintain high-engineering standards and prevent bad hires, interviewing and rejecting the author of a software that 90% of your employees use everyday is quite perplexing indeed.
While I understand that interviews are sometimes necessary to evaluate a candidate, it seems completely out of touch for Google to pass on an applicant with such an impressive background.
Not only that, but heaven knows that it takes a lot of dedication and leadership to lead an open source project from ideation to wide-spread use. If I were a startup founder, I would kill to get someone like him on my team.
In fact, I would see Google hiring the leaders of open source projects in order to influence the direction the project takes as a problem. Sponsoring an open source project is brilliant, and Google are great at that, but buying influence over it is completely different and (potentially) an issue that would lead to people forking the project.