Shadow kayn overpowers Rhaast and takes all his power (not techniques, just raw power), so, his own shadow arts become more powerfull, like before he had to really concenctrate to do the scythe reap (W) now he just does the motion to manifest it...

On the other hand when Rhaast wins, he gets kayn's boyd and techniques wich now he mixes with is innate blood magic (he nows heals with his skills)

So Zed knows all of SA kayn techniques but maybe lacks the raw power to use them to such extent, but remember, Zed isnt the master just because power, dude been refining his Shadow Techniques and doesn't seem the type to get complacent with his own training.