If you watch the video, you'll see that the encounter was at the city's performing arts center and there is literally nothing around that could be used as a flotation device. There's a boat rental place a mile away but it's 5 AM. They told him he's not allowed to stim in the lake, so he jumps in and starts swimming about 20 yards with his arms and then swims another 20 yards or so. And he's swimming towards the dam, which presumably has turbines.