Xebec was under the influence of Imu's Domi Reversi, which makes its victims almost impervious to damage, and stronger to boot. Also Conquerer's Coating has always had the ability to cancel out damage--this is part of the reason why Kaido had such strong defensive capabilities in Wano, which was exacerbated by his oni lineage. It's not new Haki or a new power whatsoever, just Roger and Garp figuring out how the damage an enemy that was possessed by Imu and using Conquerer's Coating which was clearly apparent and even stated by Roger and Garp in the chapter.
