A couple of thoughts on this now that I've had time to mull things over.
Firstly, I'm willing to bet that they're going to build an on-ramp for this adventure at about 11th level so that other campaigns (SKT, RotFM, Phandelver, etc.) that feature the mysterious obelisks can just continue on using the higher-level section of the adventure. Basically, I'm assuming that the first part of this book from levels 1 - 11 will be an adventure investigating the obelisks (heck, some from previous adventures might make appearances), with a change in scope to multiversal levels around 11 - 12, which would allow the aforesaid on-ramping from other campaigns to happen.
Secondly, it really does sound like they have heard the pleas and are looking to support, at least somewhat, higher-level play. Both this adventure going to level 20, and all the high-CR monsters they're talking about for the MM seem to show that this neglected portion of the game is going to get some love.