It's never been in doubt that this oil spill is not the biggest in world history. If Rush's argument is built on the false premise that this spill was somehow the biggest on record, and that our hero Rush was debunking this myth, then yes, he is providing "fact."
If his estimates are based on 15,000 barrels per day, then it's just a fucking guess. My response about Limbaugh making hyperbole is based on his political posturing and innate lack of actual care for what's going on in the gulf, and his spinning of facts for political nitpicking.