Muslims believe that Jesus was a Prophet. Therefore, the author of that tripe might just as well have said that Islam is the largest Christian denomination in the world.
Neither Islam nor Mormonism are Christian. They are both non-Christian religions. Just because they both claim to "believe" in Jesus in some perverted sense does not make them Christian.
The LDS Church considers itself to be a restoration of the church founded by Jesus Christ, which was later lost in the centuries after Christ in a Great Apostasy. Adherents, referred to as Latter-day Saints or, more informally, Mormons, view faith in Jesus Christ and the atonement as the central tenet of their religion. LDS theology includes the Christian doctrine of salvation only through Jesus Christ, though LDS doctrines regarding the nature of God and the potential of mankind differ significantly from mainstream Christianity.
Jews also consider Jesus to be a prophet. But neither Muslims or Jews claim to be Christians. Mormons do. So what's the big whoop?