They are the average bitrates. If you've ever watched a bitrate counter during decode you'd know that there will be sections that almost certainly exceeds 30Mbps sometimes.
It'll be interesting to see if Warners uses higher bitrate encodes, once it's encoding only for Blu-Ray. When it was supporting both formats, the lower bandwidth of HD-DVD may have been the least common denominator.
Warners stood to reap patent revenues from HD-DVD, a continuation of its patent positions in DVD. That is why initially they were supporting only HD-DVD and why when it decided to support both formats, it was such a big deal -- big enough that it precipitated MS coming out for HD-DVD.
They did try to push some kind of a hybrid format. Can't recall if it was the DVD/HD-DVD disc or some other hybrid. But the hybrid discs didn't catch on.
They were seeing their Blu-Ray releases outsell HD-DVD releases by 2 to 1. Not that big a margin though that they would switch now (Toshiba surely moved a lot of cheap players).
There will surely be conspiracy theories.