That is a lucid, logical argument. You are correct. Logically, there can be only one truth that corresponds to reality. Once the laws of logic are equally applied to LDS and orthodox Christianity, one is found wanting, to the point that it can be easily eliminated as the church by light of Scripture. Once the laws of logic are applied, here is what is found concerning the two (I added Islam because LDS follows the same model):
Islam doctrinal model: Bible has revelation, but the advent of the Quran (and other writings), they trump previously known and held divine revelation; additional revelation or interpretation can be announced by a closed group of scholars. In Islam's case, such religious rulings can be limited to regional in scope, and can be withdrawn.
LDS doctrinal model: Bible has revelation, but with the advent of the book of Mormon (and other writings), they trump previously known and held divine revelation; additional revelation or interpretation can be announced by a closed group of scholars.
Christian model: Bible is the only Scripture, there is no further revelation to men, (apart from individual revelation given by the Holy Spirit; which never comes in conflict with Scripture); there is no closed group of scholars dictating biblical interpretation: all theologians can be challenged by anyone in the church-- yes, even the lowliest believing janitor.
How does believing one knows the truth make one intolerant? That is the question. It is saying that if I believe you to be in error then I am ipso facto intolerant. Tolerance means to allow the tolerated person to live and to participate in society, not to accept the tolerated person’s ideas as being as valid as one’s own. If you are labeling the LDS as “intolerant” then it is because you don’t agree with them because I don’t think you think they must be killed.