It has to do with what your defintion of who the Lord Jesus is. LDS claim that Jesus is a "spirit brother" of Lucifer born of Elohim and not the eternal Son of God who was with God and is God.
John 1:1 In the beginning was the Word, and the Word was with God, and the Word was God. 2 He was in the beginning with God. 3 All things were made through Him, and without Him nothing was made that was made....14 And the Word became flesh and dwelt among us, and we beheld His glory, the glory as of the only begotten of the Father, full of grace and truth.
In other words, the Latter Day Saints were deceived by a false prophet according to the Bible.
1 John 4:1 Beloved, do not believe every spirit, but test the spirits, whether they are of God; because many false prophets have gone out into the world. 2 By this you know the Spirit of God: Every spirit that confesses that Jesus Christ has come in the flesh is of God, 3 and every spirit that does not confess that Jesus Christ has come in the flesh is not of God. And this is the spirit of the Antichrist, which you have heard was coming, and is now already in the world.
The term anti-Christ refers to a substitute for the real Christ.