I've double-checked, and all three are definitely flagged as DTS-ES Matrix. My receiver indicates that the rear center channel is present, and it switches to DTS-ES Matrix decoding when in "auto" mode.
All three DVDs included in the Ultimate 3D package are encoded (and flagged) in DTS-ES Matrix:
Alien Adventure
Encounters in the 3rd Dimension
Haunted Castle
The DD track on each DVD seems to be encoded in DD EX, but it's not flagged as such.
Haunted Castle in particular uses the rear...