Including the endorsement itself I can understand, since it's of interest in the context of his post-presidential career. Presidential endorsements are influential on quite a high public level, and this is the first opportunity he's had post-office to endorse somebody not his son. The specifics of what it was that was so nice about what he said about McCain is not so relevant as its potential or intended influence - so it's a stretch on the rules on original research to imply significance by quoting it.