That isn't what he said though was it. He said it took over places not that it was the most widespread. If you want to disprove his point you need to find places where Islam took a minor foothold a long time ago and still doesn't have dominance. It is you are confused because you are answering his question with the answer for another one.
Spain.
Islam was a significant part of the country for hundreds of years, before they were kicked out en masse.
Loads of them moved in recently though, and I can't think of any other examples of the top of my head.