Asynchronous reprojection doesn't literally increase a game's performance. It's purpose is to make the game feel smooth and responsive, even if your actual FPS is somewhat low (e.g., 30fps).
Think: separating the actual game rendering from your camera movements. Your camera movements are updating more frequently than the game is actually rendering at. In between the frames, async reprojection is just guessing what should be in a location.
VR games use this to help with motion sickness. The LTT video you're watching is based on 2kliksphilip's video (and Comrade Stinger's demo project), where it's used to reduce input lag.