Think: Trump tweet vs. past O actions relating to self-Pardon powers in secret. Is Trump trying to get everybody to agree that President can't do this?
The President doesn’t need approval from anyone else in the government to issue a pardon, and can even pardon someone before they’ve been formally charged with a crime, which is the key to answering this question.
source: http://amp.timeinc.net/time/4933386/donald-trump-secret-pardon-law