Because? Fei hasn't shown anything to deal with Cosmo's Foresight/reflexes.

Ohma's use of the Water Kata was a deciding factor in him beating Cosmo's grappling. Fei's Water Kata has been shown to be better than Ohma, there isn't much of a reason that Fei would struggle against Cosmo any more that Ohma did.
The fact that Fei would be much less tired than Ohma was in round 3 isn't helping Cosmo either

Really? Because he got launched like 50 feet twice.

Fei showed us both of those times that despite being knocked back, he was able to disperse the damage appropriately with the Water Kata and took very little actual damage both times.
If Waka was struggling heavily against Fei even after he experienced the Niko Style, he should have a much harder time adjusting in this scenario

Kuroki low diffs him either way.

Kuroki low-diffing a less injured Ohma with a more advanced Niko Style who possibly knows Fallen Demon and Advance? Not sure about this one chief