Ok, believe what ever you want. The Mormon cult believes in a lot of batshit insane stuff, and was started by a pedophilic con man. Personally, I think they are different enough to be classified separately, but I guess you see it differently and that’s fine.

They believe that when a Mormon man dies, he gets his own planet to be the god of with his potentially many wives. And that God is one of many in a long line of gods, and there was a beginning to this chain of gods. But also they still claim to be monotheistic. The first “star” was named Kolob, they have a hymn about it. They have their own idea of the after life, and before life, that are very distinct from mainstream Christianity. Mormons may claim to care about the old and new testament, and they do to an extent, but they care a lot more about the book of Mormon. They also have secret hand shakes, have to wear special underwear at all times*, and can get baptized for people that are already dead. Like, sure, they claim they are just Christianity with a DLC extension, but so much of their belief comes from the book of Mormon, and they put so little focus on the old and new testament that I think it is safe to classify it as its own separate religion.

You really should look more into Mormonism. Calling Mormonism Christianity is like calling Islam Christian. Both believe the old and new testament the same as Christians, but both also have their own book they view as an extension and put a lot more focus on that book. Muslims have the Quran, mormons have the book of Mormon. And a lot of Mormon beliefs are heretical to most, if not all, denominations of Christianity.