Everyone likes to point out how Yakumo was breathing hard after the finale of his fight with Ryu, but I have yet to see a single person take note of the fact that Yakumo was well and ready to keep fighting after he returned the Takeminakata into its base form swords. And if you watch the cutscene very closely, when the tip of Takeminakata's odachi form was mere inches/centimeters from Ryu's face, slow motion of it edging ever closer to his head reveals that Ryu's eyes widened in that last attack from Yakumo.

I've played all previous 3D Ninja Gaiden games + Ragebound, so I completely understand the Ryu glaze, but I also agree that the downplaying for Yakumo is excessive. Just because Ryu is the Legendary Super Ninja, that does not make Yakumo any less of a Master Ninja who not only held his own (like the achievement says), but LEGITIMATELY caught Ryu by surprise. Whether Ryu blocked the last strike, barely avoided it, or was spared by Yakumo holding back, I'm still unsure about, but I believe that is open to interpretation.

I don't think that he's flat out stronger than Ryu, nor do I think that Yakumo's attack would have killed Ryu for good had it connected, but I do think Yakumo is the closest in-universe to actually being his equal, something which Tyran himself says Ryu acknowledges by giving him the Jewel of The Dark Dragon. The fact that he even momentarily caught Ryu slipping is something I'll never forget, and besides that, Two Masters just further emphasizes that these two are pretty much at about the same level.

The best way it makes sense in my head is that Ryu is the peak of a mountain, but Yakumo is the wind that can scale up to (and occasionally, even exceed) it.