At the time Quinn wrote his book (early to mid 80s), he was a BYU prof. Or doesn't what BYU teaches serve as an extension of what the LDS head haunchos believe? Or is it that every word spoken in a BYU classroom or written by a BYU prof goes through the underground filter system at SLC HQ, thereby LDS-proofing it?
You can excuse us for not knowing that every utterance has to be officially branded with the lds.org insignia before being waved into the public airspace as genu-ine, authenticated, endorsed, confirmed, and inspected Mormonese.
When the prophet speaks and says “God said” it’s gospel, when General authority speaks and says “God said” it’s also gospel. Any body else, pray about it!
As a general rule, if it can’t be found on LDS.org, it’s not cannon of the church.