It is. It’s usually for loitering. Loitering in 2018 is more accurately known as “standing while black” or in this case, sitting while black.
It’s not that cops don’t say it to other demographics. Teenagers get it too. But even with that, white teenagers don’t get told to “move along” at nearly the same rate as black teenagers. [they’ve done undercover stuff about this through the years although i don’t know if there’ve been studies).