I think it's a conceded loss.
Think about it, the Heat's down 3-1. Now, even if LeBron pops a caffeine pill or speed or some other illegal stimulant and tries to force it to a 1 or 2 point game, he'd have to do it all over again, for game 6. And then again, for game 7. That's basically carrying a team, for three straight wins, when your opponent is vastly superior.
Thus, I believe that he decided that 5 games was enough. If this were a game 6, I think he would have stuck around, knowing that a 7th game could go either way.