User blog comment:Kaido King of the Beasts/Chapter 937: Starving for Action (and Shiruko)/@comment-27567784-20190321223346/@comment-2600:6C5D:5D00:1C9:A462:7D29:DCB0:CF64-20190323030205

The reason Zoro was thought as invincible was because of the harsh training Hawkeye put him through fighting the Humandrills.