Firstly, am I correct in stating that Warner still haven't officially released the disc specs for this title? If so, isn't it possible the notes referred to in the linked article contained errors and omissions? Probably not, I know.
Secondly, if the HD DVD disc does in fact turn out to contain fewer features than the Blu-ray disc, could it be related to capacity? E.g. the full set of features fits on a BD50 but not on an HD30, and Warner are unwilling to release a 2-disc HD DVD version.