Fundamentalist Christians and Muslims like waste vast amounts of their life memorizing their religious text and creating all kinds of arguments about them because they believe that they are the literal word of God. So when they argue they like to do so assuming that sikhs view their holy book in the same way. If you try to argue with them about it at that level you are already changing the tenets of your religion to match the type of argument they are trying to make. Guru Granth, IMHO, is not some literal word of God like the muslims say the quran is. It is a compilation of many authors who found and loved God and wrote poetry/guidance about him. Its not a science book, history book, but a teacher to help us find God. How can you really argue against it?
Have you seen those answering islam, answering christianty websites? They are filled with people who literally waste their lives arguing details of ancient manuscripts. I am not saying the research given in this article is not valuable, but its like arguing with the street preachers who visit college campuses. You got bigger things to accomplish, and if you debate them all you are doing is wasting your time. Best way to spread the sikh faith is to talk kindly to people about it, and more importantly to live morally and achieve success in life.
Just my two cents.