The mega texture was in doom3, but i don't know what form. Id & SD (i assume) have both worked extended and utilised this for QW:ET.
I think was we are see in Id tech5 is an evolution of the mega texture tech
since mega texture in this generation is only for terrain, im assume if _all_ textures in the demo are unique then surely this is a major revision on the current tech.
I think one of the key features missed is that this demo uses the equivalent of 20gb of textures (although doesn't really show it), so reading into that the new tech (imo) is very impressive, in terms of memory utilisation.