[jsmthrowaway]

That's a style guide for writers who happen to be attending Princeton, and a fairly common one, at that: generally one says John Doe, Ph.D. for a non-physician and then later says Doe, not Dr. Doe or Mr. Doe (the blanket rule against courtesy titles is also common and independent of Dr.). As for not using Dr. as a title for certain types of degrees, that is not universally accepted and has been the minority in my experience (with the exception of Juris Doctors). Pretty much every Ed.D. I have met styles themselves Dr., for example, though half of the etiquette people discourage it.

To the root point, your citation has absolutely nothing to do with a proper title for a Princeton alumnus.