So instant skills are not going off instantly - working as intended (why this then?)

[complexxL9]

I am a little confused... are people complaining that their instance skills are not going off? or they are complaining that their instant skills, which went off instantly, are preventing the use of another skill for a few seconds?

Both are true, neither instant skills are going off instantly after using another ability, nor anything else is going off instantly after using instant off gcd ability, there is awlays animation cooldown (maybe except some few abilities I don't know about).
However complaint of this particular thread is about "instants can't be used instantly, but boss mechanics require them to be used in that way". What the OP video demonstrates is that some boss spells require to be interrupted in such a small window in which it is impossible to activate interrupt skill due to how combat mechanics work.

It's like encounter designers are not in agreement with combat mechanics designers.