Dr Johnson was not using the word 'patriot' in its modern sense. If I remember correctly, he was referring to those who were inspired by Bolingbroke's 'Patriot King' - a major influence on the young George III who at the start of his reign wanted to break from the Crown's dependence on the stale and corrupt Whig oligarchs who had so dominated the reigns of his two predecessors. Alternatively, Johnson may have been referring to the rebels in the American colonies who called themselves 'Patriots'. Johnson had little time for them, asking "How is it that we hear the loudest yelps for liberty among the drivers of negroes?"