That's a nice quote, but characters, especially in One Piece, can be incorrect or just straight up lying, and that's sometimes thanks to a lack of information. Gaban's telling Zoro this because Zoro had no clue what power he was even using, but Gaban hasn't really seen him use it in action. As far as on panel showings are concerned, Zoro hasn't shown strain using AdCoC in and of itself.