The difference is that Yasuo still functions just as well off of a well placed knockup of his own, he can still get a huge ult with his own Q.
In a teamfight a Sej+Yi combo will most likely be able to stun 4-5 enemies with E, a Sej by herself will probably only be able to stun 2.
Yasuo having synergy basically improves his worst case, if you're with a Malphite you probably shouldn't ever ult less than 2 guys in a teamfight, by yourself there's times when you'll have to settle for 1, in either case you could still do considerably better. Sej improves her best case, there's always a chance that you'll just get disengaged and not get a stun, with or without an ally but with an ally you have so much better potential to stun many enemies.
I really hate this design because it ties Sej to her melee allies, even if I play the best possible as Sej I'm still not going to be able to E as many people as a retard with a Yi behind them.