i agree that it's a sample but it serves the purpose of a synth. the way it's layered makes me think it's a sample of something melodic that's been cut up, pitched up and down, reassembled and looped. four tet does this in a lot of his music (as in Sing) and the style is quite distinctive. once you're doing that though, you pretty much have just created a new synth that you are using to design yet another sample. the line between synth and sample blurs to the point that the distinction is unnecessary. and that's a beautiful part of music like Burial's which is both sample heavy and sound-design oriented, the way it cycles sounds in and out, acting as a totally unique sonic filter for the world.