Nullifying the entire EU seems unlikely. Note that the fact that the new EU books are part of the new EU seems arbitrary. They could have easily been stuff that was released before the reevaluation of the EU.

I think they just don't want to step on their toes with the new films. They're releasing a blanket statement: the story told in the EU post-ROTJ will not be the same story told in the new films. They want to get this out of the way sooner rather than later, so as to not disappoint fans when info about these films gets out.

I expect clarification as to the rest of the EU as these films solidify.