why would they have the missionaries read it if it wasn't official church doctrine?
The Church released an statement on what constitutes "official" lds doctrine a couple of years ago:
Not every statement made by a Church leader, past or present, necessarily constitutes doctrine. A single statement made by a single leader on a single occasion often represents a personal, though well-considered, opinion, but is not meant to be officially binding for the whole Church. With divine inspiration, the First Presidency (the prophet and his two counselors) and the Quorum of the Twelve Apostles (the second-highest governing body of the Church) counsel together to establish doctrine that is consistently proclaimed in official Church publications. This doctrine resides in the four “standard works†of scripture (the Holy Bible, the Book of Mormon, the Doctrine and Covenants and the Pearl of Great Price), official declarations and proclamations, and the Articles of Faith. Isolated statements are often taken out of context, leaving their original meaning distorted.
http://newsroom.lds.org/ldsnewsroom/v/index.jsp?vgnextoid=970af549db852110VgnVCM100000176f620aRCRD&vgnextchannel=f5f411154963d010VgnVCM1000004e94610aRCRDI think the statement speaks by itself.