Well, Dr. Xebec's Haki > Roger's.
So when he pierced him with that Haki and there was no scar, it means it wasn't deep enough. Therefore, we can attribute this to the fact that the attack didn't have enough power, even though it was coated in Haki stronger than Roger's.
Xebec pierced Kaido, and then hit him to push him away.
In this chapter he also hits Roger with his hands and he is visibly very affected by it; I don't see any difference in the attacks, and yet the 21-year-old Kaido was fine.
After breaking Xebec’s Haki barrier, an attack with the power of a G3, G4, or even an OKG can knock him out, because he’s not some endurance god. After breaking Kaidou’s Haki barrier, you need the attack power of a punch the size of an island — the strongest attack in the entire series so far — just to knock the guy out when he’s already on his last legs. The difference here is like heaven and earth.
That's why I'll never buy into the idea that Garp and Roger's final attacks could defeat Kaido; they don't have AP, and Xebec, after the Haki barrier, is a guy with low resistance and a fragile body.