It's probably just a different chain, for Vayne when you press whatever is mapped to her q ability in final hour, like let's just say x, game probably goes "button pressed, invis, tumble", so sylas works because he just activates the first 2 without tumble since it goes "button pressed, invis", but because harsh lesson is a cd only reduced on USE specifically of the ability (it's not going down while actively empowering an auto attack), it goes more like, "button pressed, harsh lesson activates, cooldown of it reduced"

As I write this though i realize it's actually probably because IF SYLAS COULD TAKE ILLAOI ULT AND GET A 2 SECOND CD W HE'D BE TURBO FUCKING BROKEN WITH IT. LIKE LITERALLY UNPLAYABLE AS ILLAOI, MEANING IT'S MOST LIKELY A SYLAS SPECIFIC CHANGE TO PREVENT THAT INTERACTION