Quote from fhl:
The old testament is not to be discarded. It says that "God is one" in Deut.
Beyond that,
"The Mormon Church teaches that there was a great and total apostasy of the Church as established by Jesus Christ; this state of apostasy "still prevails except among those who have come to a knowledge of the restored gospel" of the Mormon Church (Gospel Principles, pp. 105-106; Mormon Doctrine, p. 44).
If the Mormon Church believes it is the only true Christian Church, it should not attempt to publicly present itself as a part of a broader Christian community. Instead it should tell the world openly that those who claim to be orthodox Christians are not really Christians at all, and that the Mormon Church is the only true Christian Church. This in fact is what it teaches privately, but not publicly." from http://www.irr.org/mit/Is-Mormonism-Christian.html
I don't know why you think that they teach only privately that they are the only true church. Click on the link from the original post and read some of the talks from the general conference. If memory serves, almost every one of them makes a statement to that effect. The general conference is broadcast throughout the world to anyone who wants to watch it. Mormons are not at all shy about claiming to be the only Christian church with God's authority.
The LDS church does not teach that other Christians aren't Christian. They actually commend them for their belief in Christ. Albeit, they still don't have the authority to act in God's name.
As to your reference in Deut. There are a few verses that use a similar phrase. Many have taken these as confirmation of the trinitarian God. My understanding of them is that they are refering to the unity of purpose and action. If you read the original post it explains that they are believed to be undivided and unified in every way, except that they are seperate beings. This unity is what makes Christ our savior. He acted perfectly in conjunction with the will of the Father. Still, these few verses should not be used to discount more than 100 others that make reference to the seperateness of their beings.

