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Well if he submitted his thesis in 2017, he probably didn't have a PhD in 2015. Also there are several steps between submitting your thesis and getting your degree.

I suspect he may very well have considered himself 'basically done' with his PhD several years ago and just started using the title. He wouldn't be the first

Yep, that looks like it. And you can go from there to his advisor's home page and see he's listed as one of his PhD students:

http://csusap.csu.edu.au/~tzia/research.htm

Acknowledgments include supervisors/advisers.

A few weeks before bitcoin's release: https://archive.is/Ao7DQ