Yes, I agree that no right is absolute. In fact, in this case, one might be able to publish her thesis (or parts of it) without violating copyright protection if that publication falls within the "fair use doctrine." That's derived from a series of SCOTUS decisions; it's a bunch of tests that courts use to determine copyright cases typically when the defendant creates a work derived from the original work of the copyright owner.
You wrote: “Whose freedom of speech is being abridged if a candidate for public office decides not to make available to the public a book or article that he or she may have written in the past?”
Everybody’s. Look at the posts to this thread. You are surrounded by 100s of people who are now able to speak freely about Hillary’s thesis because her efforts to stifle their speech by blocking publication have been bypassed.
As it relates to a Presidential election, the right of free speech is superior to the alleged copyright.
When one becomes aware of hidden and highly controversial writings of a candidate for public office one should seek to make those available to the public. This is a very specific situation.