Disagree completely. People have many facets and many keep large portions of who they are out of their online postings.
Do I believe you could recreate Bill Cypher, yeah pretty well. I have other aspects of who I am that I hint at at most using this Nym. I have ideas and beliefs I never post about publicly under any name.
Context creates the person. We are all always a self plus a social context. I can never be the same person to my wife and my best friend even if I tried. They each bring a unique context that creates a different me.
At best I can hope the best parts of me are what gets immortalized. The panopticon necessary to collect the whole self is far far away.