If Zhou Yu had that skill, he would have started breeding horses long ago. At most, Zhou Yu knows that it can be done, but if asked where to breed them exactly and how, he certainly wouldn't know!
Truth be told, Chen Xi indeed doesn't know where Wu State bred horses during the Spring and Autumn Era, nor where Chu State did, but he does know where horses were bred in the South during the Ming Dynasty. He also knows about a breed called Dian Horse. So, Chen Xi knows the location, and as for how to breed horses, he can just copy something from Jia Xu and give it to Zhou Yu.
If it's said to be unsuitable, then it certainly might be unsuitable, but Sun Ce and Zhou Yu are very clear about the weaknesses of Eastern Wu. Now there's a chance to compensate for this weakness, even if the plan itself has minor issues or is a trap, as long as it truly resolves the problem, Zhou Yu will give it a try!
