It's entirely so. Yi and Fiora's Meditate and Damage Resist are their defensive tools for themselves, just as Yasuo's wall is his as well. It's not their only one, mind you, Fiora has her healing and Yi his Alpha Strike, but those are the tools that supply the most self defense in their kits.
It's their main form of defense, aside from their minor forms, like Fiora's healing, Yi's alpha strike, and Yasuo's shield. They're meant to protect the Skirmisher from threats, it doesn't matter in what shape or form, they're all the Skirmisher's main defense. Windwall does more than other's, because other champion's have different tools. Fiora has %health true damage, Yi as resets and immunity to slows and attack slows.
It doesn't matter the form the Self-defense takes, exempting one self-defense from Ults, is purely in the camp of people hating Yasuo deleting their ult, not in the camp of making it fair for both sides. Ironically, you create a situation that's unfair for Yasuo, his main defense not working when he'd most need it.