What do you mean by that??? IIRC, Voltaire et. al. were arguing against the kind of church-state commingling that was 18th Century France.
Voltaire saw nothing like the St. Bartholomew's day massacre in contemporary Islam. The fact that Turks could rule over Christians and Jews without forcing them to convert or massacring them contrasted pleasantly with some of the events of the 16th and 17th centuries in Europe.
Voltaire despised religious orders, such as were common in Catholic lands. The lack of clergy-laity division that your author notes in Islam would have appealed to him. He also disliked the stress on miracles in Christianity, and the underemphasis on miracles and metaphysical speculation in Islam also attracted him.
Voltaire did write a play about Mohammad and fanaticism that even today earns him the condemnation of Muslims, though his true target may once again have been the Catholic or Christian church he could not attack openly. But like other Enlightenment philosophes, he sometimes used Islam as a foil to bring out what he took to be the vices of the Christianity of his day. The same Muslims who attack his play, cite his attacks on the Trinity.
What comes out of the small amount of research I have put into this today, is that Europe of the early modern age was not only more intellectually inquisitive and economically active than the Islamic world of that time. It was also a good deal more ferocious about religious and political questions. Maybe Islam slumbered under a repressive and deadening blanket of passivity and conformity. But the West must have looked frightening to some people in those days, including to many who lived there.
Another thing that comes out is that many deplore that Islam never had a Voltaire. If and when it does, we can imagine that he would be more friendly to Christianity or Judaism than his co-religionists, as Voltaire himself found positive things to say about Islam that the Christians of his day would not.