Are you referring to SACD's that are purely stereo discs, or surround SACD's that you want to listen to in stereo mode? If you disable Pro Logic II on the BDP-83, the multichannel analog output will provide you with unaltered stereo playback of a stereo source (stereo SACD, CD, etc.) and you can rely on just the multichannel analog output for everything.
I'm not sure where you came across that or exactly what the person was referring to. The BDP-83 does have a speaker configuration option for "downmix" that converts multichannel sources to stereo, but that setting doesn't really affect stereo sources. It also has a setting for SACD priority that defines which SACD layer or track it should default to (options being CD for the CD layer of a hybrid disc, stereo for the stereo track on SACD's with both stereo and multichannel mixes, and multichannel for SACD's with multichannel mixes), but that doesn't create any downmixing. It just determines which separate version of the disc gets played.