Actually,
Brigham Young taught that the moon was inhabited during a sermon given in the
Tabernacle in
Salt Lake City in 1870. His words were later printed in the
Journal of Discourses, vol. 13, p. 271. If that's not acting as a
prophet, what is?
For every quote by a Mormon attempting to excuse the words of a prophet by insisting he wasn't acting as a prophet at the time he said something, there is always another Mormon ready to state that we need to follow every word that comes from a prophet's mouth. There is a good amount of muddlement on this issue, and in practice most Mormons end up not even researching the disturbing things like this that Brigham Young or other old prophets have said, since "a living prophet trumps a dead one", and it's easier not to deal with disturbing issues.
"Anti-mormons" actually do have a lot of "ammo" to bring against the LDS church, especially on scriptural sources (see Book of Abraham and Book of Mormon). It's just that most Mormons refuse to consider much ammo to be real ammo and are adept at providing excuses for issues they find troubling.