Most Christian Denominations recognize each other as Christian, but they are most entirely united in rejecting Mormonism, because Mormonism has distorted who God is, who Jesus is, and the nature of salvation to the point that it's hardly recognizable.
Mormonism did the same thing that Islam did. Some guy announced himself a prophet and built a base of believers in himself, so that he could have as many women as he liked and enjoy the riches and power of being head of his own church.
We actually could turn a blind eye to the magic underwear or dietary restrictions as long as they didn't make them a condition of salvation. But a different God and Jesus and a different plan of salvation? There is a point where you really have to say, are you going to allow anything and everything to claim to be Christian no matter what they teach?
I think I misunderstood who you were directing that comment at. But my objection to Mormonism is much more than them claiming they are the “only” Christians. It’s what they teach on core doctrines, that ban them from Christianity.