it was not capitulation by the Christians but rather ensuring that a victory could be retained
The author is also incorrect that Europe had technical superiority to retake constantinople in the 1500s. In the 1700s it did, not earlier
Sure, it was brutal back then, but not as brutal as we imagine ~ those people were tough, and the same things we find exceptionally brutal today were, given the conditions, considered tolerance and love by those folks.
There were also class differences. The guys at the top were always treated differently than the guys at the bottom. Jews were always treated differently than Christians or Moslems.
Then there were the trouble makers who always wanted to do a voluntary conversion so they could protect their economic advantages ~ that ended with Ferdinand and Isabella. Since the only place left to conquer was Morocco, that was considered an end-time and folks were left to become Christian or leave the country.
All things considered, the best place in Europe to live North of the Mediterranean from about 700 AD to 1660 or thereabouts was called Spain ~ or, if you didn't want kids, Venice. Otherwise it was pretty much like the worst third-world he**holes around in modern times.