This quote taken from the link I provided makes it sound like it does have a core 5.1 track...
..."By contrast, the Blu-ray format mandates Dolby Digital, while Dolby Digital Plus is optional.
This means that Blu-ray Disc players might only be equipped with a basic Dolby Digital
decoder on board. If a Blu-ray disc carries Dolby Digital Plus (or Dolby TrueHD) bitstreams,
there will also be a companion Dolby Digital track to ensure playback compatibility with every
player configuration. Rather than transporting a core 5.1-channel Dolby Digital bitstream
and a separate 7.1-channel Dolby Digital Plus bitstream on the same disc, the Dolby Digital
Plus bitstream will be constructed as a hybrid of a 5.1 Dolby Digital core frame followed by a
Dolby Digital Plus extension frame. A basic two- or 5.1-channel Blu-ray player only needs a
conventional Dolby Digital decoder IC to play these bitstreams. An advanced player with 7.1-
channel decoding capability will include a Dolby Digital Plus decoder that will decode both
frames to reconstruct the 7.1-channel program as depicted in Figure 4."...