I didn't know that. If it's true then it's certainly a point in favor of my Evolutionist argument. However, if they can still breed with the ancestral Coi then they are still the same species, as dogs and wolves are. They haven't diverged enough to be considered different species.
This is also treating speciation as a black/white issue. One prediction that evolution makes is that there should be cases where it is difficult to determine if a new species has formed or not. For example, are chihuahuas and great danes the same species? One widely used definition of species is that two organisms are the same species if they can successfully interbreed. Can a female chihuahua really successfully mate with a male great dane? Can it successfully give birth to great dane/chihuahua hybrid pups? It is questionable as to whether this could occur or not. It also points out another weakness of this definition of species, namely that the ability to interbreed is not a transitive property. That is given that organism A can interbreed with organism B and that organism B can interbreed with organism C, it is not strictly true that organism A can necessarily interbreed with organism C. (think of A=chihuahua, B=some medium sized breed of dog, C=great dane) Would organisms A and C be the same species or different ones? Going strictly by the successful interbreeding definition, they are not. There are several examples of this phenomenon, and it is exactly what we would expect if evolution is the mechanism that produces new species.