I think I’ve figured out what’s going on here.
Catholics are using a different definition of the word “Christian” which is contrary to that which is used by the secular world and Mormons themselves.
Merriam-Webster defines Christian thus:
a :one who professes belief in the teachings of Jesus Christ
Catholics define Christian this way:
One who is a Baptized member of the Body of Christ.
So using the secular definition of a Christian, Mormons ARE Christians. Even Bahai’s and Muslims could be shoehorned into being a Christian using this broad definition.
But theologically speaking, from a Catholic POV, Mormons are not Christians - they are Abrahamic Theists.
So it depends on how one defines the word “Christian”.
I personally would concede to the LDS that they are Christians according to their own belief and secular definition, but they are not true Christians theologically speaking from a Catholic POV.
Mormons ARE Christians in a broad, secular sense.
Mormons are NOT Christians in a narrow, Catholic theological sense.
I hope this helps clarify this issue.
Also, in our dealings with LDS members, the charitable thing to do is call them what they call themselves - Christians.
Amongst ourselves or in technical theological discussions, we can refer to them as Abrahamic theistic non-Christians.