Many 'mostly engineering' companies, including Google, have had and do have entire offices for specific non-engineering roles, such as marketing or whatever, anything which isn't seen to be needed to be in the same location, or perhaps there's a better 'hot spot' for, say marketing in NYC and engineering in SF.
"This information is most likely buried in Google’s annual report — the last one I could dig up is their 2015 fiscal year end report. According to the paper published by Google Inc and Alphabet Inc, there are 61,814 full-time employees with 23,336 in research and development."
Bear in mind R&D isn't pure developers. It also includes QA and project/program management roles.
He cites numbers from Google's previous financial reports (likely to be valid) and then adds a guess of 45% at the end (less likely to be valid). If you want to believe the guess, well, that's up to you.