Facts:
- Adventure Zones will offer content for groups from four to 24 players, some of that content specifically designed for 24 player groups
- Adventure Zone encounters are less single boss monsters and more large battles that require your group to split up and time their actions
Source, Paul Sage (ZOS): http://www.buffed.de/TESO-The-Elder-Scrolls-Online-PC-239823/News/The-Elder-Scrolls-Online-Creative-Director-Paul-Sage-aeussert-sich-zur-Item-Progression-zu-Mounts-und-zum-Gildensystem-im-Video-1072227/
This contradicts your OP me thinks 
My personal speculation on Adventure Zones (no source, obviously):
- I have a feeling that one or more Adventure Zones WILL be in at launch. We know that they are working on them, and with the high focus on raid content in the community I am certainly that AZ's have high priority in the dev team. However, they never promise that content will be in at launch until they are actually ready with said content. Same thing happened with First person view. They used to say they work on it, but it will not be in at launch. However, we now know that FPV will be in at launch. I have a good feeling that at least one Adventure Zone will make it into launch as well.
- Another thing I have a good feeling about is that we will see several types of Adventure Zones. Maybe not at launch, but down the line as new content is added. I think that we will see both open world Adventure Zones, which have already been confirmed, but I think that we will eventually see instanced Adventure Zones as well for more traditional raiding for those who have demanded that.
Just my two cents on the matter.