At the time he did the 16 minute breath-hold (on oxygen), the existing record was held by Tom Sietas - a legitimate freediver with multiple world records. Sietas had built his record gradually over a couple of years - 15:02, then 15:16, then 16:14.

The likelihood that Blaine comes along and beats a guy like Sietas is very low. A giveaway is that his heart-rate stayed high throughout his stunt; to beat Sietas without even lowering your heart-rate is very far-fetched. It would be like if he beat Usain Bolt in a race and his ecg showed his heart-rate stayed at 70bpm. Just makes no sense.

He is a magician first and foremost so some trickery is always on the table.