Well, I do not hear Mormons saying that either. Most of us will acknowledge others as Christians, even as we disagree with them on particular points of doctrine.
But Joseph Smith did say that and in doing so excluded himself from having a broader community.
I think Joseph Smith was far less rigid about this than you suppose. He objected to the various creeds of Christendom because he thought them to be too limiting:
I cannot believe in any of the creeds of the different denominations, because they all have some things in them I cannot subscribe to, though all of them have some truth; I want to come up into the presence of God, and learn all things; but the creeds set up stakes, and say, "Hitherto shalt thou come, and no further;" which I cannot subscribe to. (History of the Church 6:57).However, Joseph Smith was happy to acknowledge that other faiths possessed truth. In fact, he said that good Mormons must seek out and acknowledge truth, wherever it is to be found:
Have the Presbyterians any truth? Yes. Have the Baptists, Methodists, etc., any truth? Yes. They all have a little truth mixed with error. We should gather all the good and true principles in the world and treasure them up, or we shall not come out true "Mormons." (History of the Church 5:517).Furthermore, Joseph Smith strongly believed in freedom of religion. He said,
The Saints can testify whether I am willing to lay down my life for my brethren. If it has been demonstrated that I have been willing to die for a Mormon, I am bold to declare before Heaven that I am just as ready to die in defending the rights of a Presbyterian, a Baptist, or a good man of any other denomination; for the same principle which would trample upon the rights of the Latter-day Saints would trample upon the rights of the Roman Catholic or of any other denomination who may be unpopular and too weak to defend themselves." (Documentary History of the Church Vol.5, p. 498.)Gordon B. Hinckley, the current President of the Church, has urged us to respect those of other faiths:
Be respectful of the opinions and feelings of other people. Recognize their virtues; don't look for their faults. Look for their strengths and their virtues, and you will find strength and virtues that will be helpful in your own life. (Dew, Go Forward with Faith: The Biography of Gordon B. Hinckley. Salt Lake City: Deseret Book, 1996)
President Hinckley has also said,
There is no need in any land for conflict between diverse groups of any kind. Let there be taught in the homes of people that we are all children of God, our Eternal Father, and that as surely as there is fatherhood, there can and must be brotherhood. ("Four Simple Things to Help Our Families and Our Nations," Liahona (June 1996): ),
In bearing testimony of Jesus Christ, President Hinckley spoke of those outside the Church who say Latter-day Saints ‘do not believe in the traditional Christ. "No, I don't. The traditional Christ of whom they speak is not the Christ of whom I speak. For the Christ of whom I speak has been revealed in this the Dispensation of the Fulness of Times. He together with His Father, appeared to the boy Joseph smith in the year 1820, and when Joseph left the grove that day, he knew more of the nature of God than all the learned ministers of the gospel of the ages. (Church News, week ending June 20, 1998, page 7)
From the lips of your own president...we are not talking about the same Christ...That puts us in two different camps.
Is this statement SCRIPTURE?
Does this extend to those who DO practice polygamy today and claim to be Mormon?