When consulting the Martial Tome the skill "Sky Kick" should be available from level 15 onward.
Alas when i entered HongMoon Training Room before level 30, i could not pass the "Status - Airborne" training (the 4th Basic Training) since
Sky Kick was not available to my Kung Fu Master.
This changed once i reached level 30. Sky Kick then worked as intended.
It looks like Sky Kick's prerequisites are still adhering to the "old" skill tree.