That's incorrect. Oda said that, while Luffy's CoC knocked out half of the Fishmen, either Shanks' or Rayleigh's CoC would've knocked out all of them. Doesn't mean Shanks' Haki = old Rayleigh's, but that both Shanks' and Rayleigh's would be enough to knock out all of them. He mentioned Rayleigh along with Shanks because they were the greatest references at that point and also similar to each other.
Obviously, prime Shanks' CoC is > old Rayleigh's, arguably even prime Rayleigh's, as Shanks is being portrayed as the new gen Roger. Though obviously, Rayleigh is close to Roger anyways.