So, georegistration is what makes GeoPDF GeoPDF?

Well... maybe yes, probably no. Or vice versa. It's become confusing. When TerraGo products create a GeoPDF file, they do more than just take data, make a picture of it, and stuff a little georegistration payload in the result. For instance, as Adam has pointed out, we take advantage of PDF features to provide a richer, more interactive, PDF file. However, many features often associated with GeoPDF are not unique to GeoPDF. Layers are not unique to GeoPDF. Object data are not unique to GeoPDF. These are "just" features of PDF that anyone with the time and patience can implement. ESRI, Bentley, Safe, and Cadcorp have all done it, although I'm not certain they all embed georegistration payloads.

In my mind, it's not GeoPDF without georegistration, that's for sure! As we work through the history and some of the implementation details in the history of GeoPDF, maybe we can sort this out to everyone's satisfaction.