Have you considered that Riven's abilities aren't based on the same teachings as Yasuo's? Given that her upbringing seems almost militant, I doubt she had the time to go off and train with some reclusive hermit master of wind techniques. On top of that, if she were using the same sort of techniques that Yasuo employs she would be much less about brute force and much more about finesse. She even utilizes Ki, a spiritual power that has absolutely nothing to do with any kind of wind techniques, to deal a burst of damage radiating out from her and, from what it looks like to me, as her shield and what she uses to dash forwards when using it.
Riven is many things, but she is far from dainty about how she goes about dealing out retribution to those who tower dive her. She's brutal, unrelenting, and while she may appear mobile that's only because her abilities are literally throwing her around through brute force, not because she's flowing with the wind to move.
So far as I can tell, Yasuo is surprised to see someone using techniques that resemble the teachings that he went through, but nothing more. Riven isn't really the type to have killed a master for no reason given that it's the mass loss of life during the war that made her abandon her own people in the first place. She's against the senseless loss of life, and she has respect for those who fight honorably. She would have no reason to kill Yasuo's master.
The much more likely story is that it has something to do with Ao Shin, the yet unreleased wind dragon champion that was teased at about two years ago. Plenty of champions say special things around other champions that have nothing to do with lore and more to do with their personality. Gnar will mock a Garen by shouting out "DEMAGLIO" but he isn't an enemy of Demacia, he's just having fun.
Yasuo is just noticing some odd similarities between his own style and Riven's. I'd be surprised if he didn't, but with Riven a fallen warrior seeking redemption the idea of her taking the life of an old master doesn't sit well in my opinion.
