This guy (reviewing the GOUT release) seems to think it's Pro Logic matrixed:

The audio here is plain old Dolby Digital 2.0, the exact same audio track included on the 1993 Definitive Edition LaserDiscs converted to Dolby Digital. It includes the same Pro Logic matrix cues that the 1993 LaserDiscs had, so today's Pro Logic II decoders will have an easy time with it, but I really would have liked to have seen an uncompressed PCM stereo audio track included as the primary audio track. Since we're being limited to two channels anyway and these discs are essentially a sort of "archive" copy of the versions that Lucas has now deemed to be simply rough drafts, it would have been a nice touch. It would also have been nice to see a Dolby Digital 5.1 track derived from the old Pro Logic mix as a secondary option for folks without Pro Logic II decoders.

http://www.prillaman.net/starwarsdvdreview-trilogy_le.html (read down)