User blog comment:Uraharattack/Who's Backing Whom?/@comment-34981281-20160210005918/@comment-24850639-20160211175436

In this sense, he's using the term "Supernova" to refer to a relatively young pirate with a moderate-high level of power and influence. Cav and Barto are not part of the Worst Gen, but they were definitely Supernovas at a point in their career (Barto is one currently).

Also, they may not be playing for themselves, but they are still playing the game. They now officially serve under Luffy so they are part of the entity that comprises "Luffy". Just like Zoro, but unlike Law, who has his own ambition separate from Luffy.