You used the words: "the greatest conquerer" - it's just this that's doubted, not that the conquest happened.

Historically, in terms of area, or percentage of (world) population involved, there are strong contenders for the title, other than the one you mention. Not to mention, some capital cities are stuffed with treasures they looted a millennium ago - so if you mean "the amount of conquest summarized over history" then your "greatest conqueror" doesn't seem that special at all.