I don't but my guess would be that he thought the message of the Koran was fairly straightforward - none of these implausible complications about women being impregnated by a Holy Spirit - but that Muslims themselves behave badly. I don't know if he had any particular example in mind of the latter. He liked to come up with paradoxes that caught people's attention. It's a bit like saying football is a gentleman's game played by hooligans and rugby is a hooligan's game played by gentlemen (someone said that but I don't think it was Shaw).