You've made an excellent point!Mormons are only "Christians" by proxy. The Qur'an and the Christian Bible was an expansion and edited work of the Jewish Tanakh. The Book of Mormon is a many times edited expansion of the Christian Bible. Calling a Mormon a Christian is not much different then calling a Christian or a Muslim a Jew. There are a lot of commonalities but a few big differences that drive a wedge.
But using your logic of "differences", that would make modern Christianity a proxy to ancient Christianity. There are commonalities but many big differences. Modern Christianity is as different to ancient Christianity as Mormon Christianity is different to modern Christianity.
An excellent point you've made.