What an ignorant thing to say. Truth is defended daily by those who do not understand. If you never had to defend truth, innocent people would not have to go to court.
You can only claim it is fiction if you can prove the Mormons' claim of it being true is false. Can you prove it? No. You also can only state your claim. Also, for your information, the text of the book has never changed since it was first published. Only minor punctuation has been changed.
For all interested in any religious book: as it says in the New Testament, by your fruits you will know them. If the book and the Church that holds it as true bring forth good fruit, then it is good. If it brings forth evil fruit, then it is bad. You decide, but take care. If the fruit and the church are good, then you should investigate more or you will be denying the truth by choice.