[Gameplay] bug kalista GA

I was playing kalista against amumu on top lane, and amumu pop my guardian angel the problem is that during the death time of ga, i could attack him and i killed him. I'm not sure about that but normally when ga pop, you can't do anything during 2 seconds more or less untill you "respawn" from the ga. please riot tell me, bug or not ?!

ps : i'm sorry for the english ... http://matchhistory.euw.leagueoflegends.com/fr/#match-details/EUW1/1954619051/36124900?tab=overview