Well, they believe in Christ: his birth, life, teachings, and resurection. How then, are they not Christians?
Sure, they believe that a tribe of Israelites came via boats to the US, had big battles with swords and chariots, and lost to the unrighteous who were then "cursed with dark skin".... and that all of this has no historical evidence. It may be all made up by Joseph Smith.
However, those stories and beliefs don't invalidate their belief in the eternal Christ. I believed in the tooth fairy as a kid, but I still believed in Jesus. Catholics asked to St Christopher pray for them, but he was downgraded by the Vatican due to lack of historical evidence. Did those who relied on a possibly non-existant St Christopher have their faith in Christ invalidated?
Bottom line is that despite questions, Mormons are Christians because they believe in Christ.
Using this same logic; how would you describe THESE guys??
So does Satan