Endermen take too long, or are unable to, teleport away from danger, especially if there are few available blocks to teleport to.
1. Build the setup shown in the video, in the end, far away from any end stone islands.
2. Spawn endermen into the water.
3. They take a long time, or even are unable to, teleport away from danger.
Endermen should be able to immediately, or at least in a reasonably short amount of time, find available blocks to teleport to if they're in danger.