Not sure if I ever get any answers here, but as I mentioned it was so long that I can't remember this clearly.
Maybe it was actually only the Sky Kick->Rising Kick->Falling Star that was in "Contextual Skill Slot 2" used by holding RMB and Guiding Fist was "Contextual Skill Slot 3" bound to "F Key", I'm sure however that it was certainly not used by pressing "2" on the keyboard.
All I can remember is that it was already after the changes to the game where they removed skill tree that allowed you to make your own build and split the KFM Marital Tome into separate sections for Dragon Fist, Flying Kick and Iron Claw instead.
This leads me to question if the skill enhancements trough Talents are even worth the Martial Tome Page Expansion, or all the Page Expansions I got back then are wasted money on NC Coins now.
Nonetheless, back to the main topic. The only conclusion I can get to on my own is that this change was made because aerial attacks are very unpopular in PvE as it prevent some attacks to reach the enemy, thus breaking the max DPS rotation, so they moved the aerial attack to other key where it's less intuitive to use and I probably should learn to not use the Sky Kick at all, with some possible exception in PvP.