I think the idea is that a lot of the volume is gas rather than oil. When you watch the video you can see the oil/gas mix change every few seconds as the plume changes from black to silvery bubbles. I guess a "natural gas slick" isn't as bad as an oil slick.
New Scientist has a discussion of the amount
here; the answers generally fall in the 50 to 100 thousand barrel per day range.
Right now the thing to do is stop the flow, but let us not forget: eleven people died on that rig, and the blow-out preventer was supposed to prevent that, too.