I think he's talking about Miers. His comparison is on judge vs. non judge, not CJ Rehnquist vs CJ Roberts or Rehnquist-replacement vs O'Conno-replacement.
"... the conservative legal icon said that the High Court needed someone who had never served as a judge to take the place of the late Chief Justice William Rehnquist. Sandra Day O'Connor."
That would be more accurate, IMO. After all, Miers is not going to be Chief Justice, as was Rehnquist.
He's not referring to Roberts as Renquist's replacement as CJ; he's referring to Miers as replacing Renquist as a justice who came to the high court with no prior judicial experience.