The argument goes something like this:
The word “Lucifer” in Isaiah 14:12 presents a minor problem to mainstream Christianity. It becomes a much larger problem to Bible literalists, and becomes a huge obstacle for the claims of Mormonism…The first problem is that Lucifer is a Latin name…Lucifer wasn’t equated with Satan until after Jerome [in the fourth century BC]. Jerome wasn’t in error. Later Christians (and Mormons) were in equating “Lucifer” with “Satan”…Mormons claim that an ancient record (the Book of Mormon) was written beginning in about 600 BC, and the author in 600 BC supposedly copied Isaiah in Isaiah’s original words. When Joseph Smith pretended to translate the supposed ‘ancient record’, he included the Lucifer verse in the Book of Mormon. Obviously he wasn’t copying what Isaiah actually wrote. He was copying the King James Version of the Bible. Another book of LDS scripture, the Doctrine & Covenants, furthers this problem in 76:26 when it affirms the false Christian doctrine that “Lucifer” means Satan. This incorrect doctrine also spread into a third set of Mormon scriptures, the Pearl of Great Price, which describes a war in heaven based, in part, on Joseph Smith’s incorrect interpretation of the word “Lucifer” which only appears in Isaiah.
The short answer is that much of what is published at lds-mormon.com is incorrect. The longer answer is given by Ben McGuire (see Lucifer and Satan). Go read it, then feel free to discuss it here.