Sorry, but I believe Johnson and Burke were making the same point - ie the mere fact you keep slaves doesn’t diminish your own jealousy for your own liberty
Johnson was arguing against those who said that England’s commitment to (jealousy for) its own liberties would be weakened if England kept treating the colonists as, in effect, slaves. He said this wasn’t so, and he pointed out that the colonists were keeping slaves without weakening their commitment to their own liberties
I see what you are saying now. Essentially that short quote from Johnson has been taken out of context.(not surprising) When I initially read through the essay, that escaped me.