Bear in mind that recent world populations are much higher than before the Reformation. While the percentage of Christians who ever lived who were also Catholic is quite high (probably 75 to 80 percent), it certainly isn't 99%.
Just nitpicking...
Prior to the reformation the Christian population had expanded and contracted several times due in part to plagues and conquest. The number is valid. However, even if, for arguments sake, the number were reduced to 89% or 79% the point is still valid.