The explanation is based on a misunderstanding that the person asking him the question was experiencing some kind of Dolby Surround or Dolby Pro Logic decode of the 2 channel mix on the SACD.
Steve didn't understand that there is actually a decoded 4.0 extraction (not going to call it a separate "mix" any more) on the SACD.
Actually I only needed the very first part of that explanation..."there is no 4 channel master"...which for my purposes makes it just a pseudo surround effect...which can probably be achieved by a receiver setting and bested by a surround master..