If the song were genuinely traditional would he have claimed sole writing credit on the single?
Personally I like to think not.
Frankly it may be true that this song is (more accurately may be) in the process of becoming traditional. However this would hold true in all similar cases E.G. the works of The Arctic Monkeys.
We will all have been a long time dead before the word Traditional can be meaningfully used in respect of contemporary music.