What are the nodes?
you have to intercept in that fight I’m pretty sure while he’s unstoppable
Use Chee'ilth if you have her. Sp1 shuts him down.
It's a 20-second Unstoppable, and only has about five seconds left. So it's been running for about fifteen seconds.
The SP2 inflicts 30 Decelerates, expiring at 1/second; so you've hit him with an SP2 less than four seconds ago.
So, the answer is that you've only just inflicted those Decelerated on an already-Unstoppable opponent. The mechanism of action for Decelerate is to prevent Unstoppable activating through AAR. Decelerate can't achieve anything if an opponent is already Unstoppable.
To expand on (and kind of prove) the above answer, Slow is different from Decelerate:
See - Slow has two actions: it reduces the ability accuracy of Unstoppable and it has the same effect as Grit - making Unstoppable champions react at though they weren't.
So Slow works, even if the opponent is immune to AAR and/or is already Unstoppable
Decelerate doesn't have an equivalent ability to deal with Unstoppable that's already active.