The 2 million gallon figure is just a sarcastic joke.
What was basically meant that if the animal rights activists could put a limit on what size tank/pond that private aquarists could keep something like a Blacktip Reef at - it would be some crazy figure, like 2 million gallons. But the thing is laws don't work that way - so they can't directly set those restrictions.
However if it ever becomes law - that sharks like BTR can't be kept by private aquarists without a special permit. Then I'm sure that get said permit - the private aquarist would need a tank/pond which is 25,000 gallons, with a footprint of at least 700 sq.feet. Basically to prove that they can safely keep those sharks for their entire lives.
Bigger is Better - especially with shark tanks