It is an approved copy of a speech by Mormon President Brigham Young in the new tabernacle at Salt Lake City, Utah, on October 30, 1870. It was taken down in shorthand by one George Watt, his clerk, and published. I would not presume to argue with such an august fellow in the LDS church about what their beliefs are.
They officially don’t consider it canon.
Though Brigham Young said, “I have never yet preached a sermon and sent it out to the children of men, that they may not call scripture” (Journal of Discourses 13:95), the next line is, “Let me have the privilege of correcting a sermon, and it is as good Scripture as they deserve.”
So they say that since he never had the chance to correct these sermons (even though there were thousands of them over several years), they can’t be considered scripture.
And heck, if things get too sticky, there’s always the retroactive revelation trick. :-)
Not David W. Evans?