Requiem ultimate cooldown increase (or unaffected)

I was playing a match of ARAM as Karthus and noticed the cooldown on my ultimate not being affected by any CDR I've built. I was able to build up to 45% using the following build:

Sorcery: Arcane Comet The Ultimate Hat Trancendence Scorch

Inspiration: Perfect Timing Cosmic Insight

Items:

Here's a video I quickly clipped from the match. Check the description. https://www.youtube.com/watch?v=UKXspxwprnA&feature=youtu.be

I understand it's low quality as I have just uploaded it. If you would like to check the replay for yourself you may pull it off my summoner profile/match history.