That many were killed is not disputed. Rather the context as reinterpreted by modern people. As the author says:
Many were killed, yet many others were ransomed or allowed to go free. By modern standards this may seem brutal. Yet a medieval knight would point out that many more innocent men, women and children are killed in modern bombing warfare than could possibly be put to the sword in one or two days. It is worth noting that in those Muslim cities that surrendered to the Crusaders the people were left unmolested, retained their property and were allowed to worship freely. As for those streets of blood, no historian accepts them as anything other than a literary convention. Jerusalem is a big town. The amount of blood necessary to fill the streets to a continuous and running three-inch depth would require many more people than lived in the region, let alone the city.
By modern standards this may seem brutal. Yet a medieval knight would point out that many more innocent men, women and children are killed in modern bombing warfare than could possibly be put to the sword in one or two days. That is your defense for the Rape of Nanking? or do you see a difference between bombing losses, versus putting women and children to the bayonet, and the knife?