Zac and "Hard CC".

It is a knockup, so yes, it is obviously hard CC.

they can't act during it so ya

slows and roots (soft cc) still leave the player with options to act, while knockups/displacements/stuns stop them from doing anything (hard cc)

Zac's E and R can both proc it.

This makes the Colossus Keystone potentially very, veeery attractive on him, doesn't it?

Especially late game when initiating onto the entire enemy team.

Anything that knocks about counts. Hell, Quinn's E is counted as a knock back :P

Yes

Soft CCs are anything that let you still do skills/move freely

For instance a slow and root arent considered hard CC They are soft because your champ is still able to do something.

A Hard CC is /stun/knockup/knockback/Supression/Fear ( Where you have total loss of your champ )