Xebec doesn´t need to know about the Nika fruit, Joyboy isn´t his ancestor, Davy Jones is. Davy Jones is obviously as strong as Joyboy or even stronger. If he used Haki to harm Imu, the Davys would know about how a top haki user could harm Imu and talk about it, but instead we only learn about the 2 devil fruits. It´s obviously not a coincidence that the most important davy in the manga spent 30 years or whatever looking for a special DF. Not training for ¨muh Soyboy level Haki¨. And again, Xebec isn´t a real person, Oda is just using him for exposition.
Headcanon, the fact that Imu can completely ignore several ACOC attacks implies he has a seperate power that haki can not harm. Joyboy isn´t shown to be an exception, and the most famous thing about him is his god fruit, with 2 others existing. One obviously for Davy Jones and the former Giant King
Your clown ass literally thought that Xebec was looking for the Nika fruit as one of the 2 devil fruits like 1 month ago lmfao.