But again, Symington had had a specific indictment made against him, as Nixon had against him (in the preparation to the impeachment process). It might be possible to see how a pre-pardon could apply to a past indictment.
However, Comey didn’t push for indictment. A pre-pardon couldn’t refer to any accusation here. This might have been a clever way of kicking the can forward to Donald Trump.
Unless the Obama administration, upon Hillary losing the election, wants to Kabuki-theater an indictment of Hillary... I don’t see how this is out of the authority of a Donald Trump administration.
I expect Hillary, like Assange, to flee the country to a non-extraditing place.