"Rookie Xebec killed Admiral" is maybe the same case with his son: "Yami only Teach killed Whitebeard".
We didn't know the context how it happened.
On another side I can smell double standard clowns already. Those clowns will 100% believing "Teach will surpass Xebec the Admiral slayer", just like they believe Teach will surpass prime Whitebeard, prime Roger, Imu, etc. Doesn't matter it is wrong or right though.
But the same people will automatically rejecting the idea of Shiryu surpassing Mihawk. Or Shiryu surpassing Oden.
"But but Shiryu is just CoO training tool"