As for slaughtering the jewish clans systematically I've read something a bit different, which was more along the lines of: After they dismissed him and ridiculed him he fought them and all the male members (warriors) of one clan were killed while the others who foughth him were exiled.
PS: Not a direct quote, just roughly what I recall reading.
Anyway I share the views of many that what this books says should not be taken seriously as he's just trying to inspire hatred against muslims, the very same thing he's blaming Mohammed for doing against the jews.