They say they are Christian.
If you want to write the definition to exclude them, feel free to do so, but that is a quixotic undertaking. Christianity as a concept is broad and fuzzy. There are “sort of” Christians all over.
Pope John Paul II says they aren't, but having seen many Catholics on Mormon threads, it seems that he and Catholic teachings don't carry much weight among pro-Mormon Catholics, who first loudly announce that they are Catholic, and then proceed to fight like tigers for Mormonism as a Christian church.
Jesus answered, “The work of God is this: to believe in the one he has sent.”