He did not denounce islam, he denounced the uses and interpretation of islamic teaching that power the terrorists and allow their followers to do insane things in the name of a god that he continues to believe in. Asking someone to give up his or her religion is a tall order no matter what the religion is or is made to be.
That said, I would like to know just when this was written, what has been written since, and what reactions it has caused....my own problem with islam, of moderate muslims, is that the followers (today) simply cannot rid themselves of the underlying attitudes toward 'others' and that those self justifying prejudices always seem to be hiding just below the surface - no matter how much contact they have with non-muslims.
See at another thread, "Wafe Sultan" (travels to Israel) regarding indoctrination and how long it might take to reverse it.
His illness is islam, and it is completely self-induced.