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