Yes, John Mueller is an authoritative and official representative of Google. No, John Mueller is not all knowing and infallible.
John is one of the Google voices to partially help to communicate what the 88,000 full time Google employees are doing. The secret Google formula has many moving parts that are constantly evolving. Even if one person knows all of those pieces, that Google employee would not give away the secret formula because they work for Google and not for us.
As webmasters we need to remember he is speaking in very general terms to the billions of websites out there. I am 100% confident that he is speaking on behalf of Google's interest and not necessarily in the interest of optimizing our profits. Obviously each of our individual websites operate in their own specific circumstances so we need to intelligently apply his general advice.
Regardless if you like or value John's comments, you should realize that he does go above & beyond to work with webmasters. He has given up a good chunk of his unpaid personal time to connect with webmasters.