Having listened to the whole song again, his singing ability has almost certainly gone, at least on this evidence. Either the song was too high for him, or he can't sing anymore. The sad thing is, Bowie will be in danger of being remembered for this song in future years. .his big comeback song after years of being in the musical wilderness, not for all the really quite good stuff he did in the 70s and 80s.
The whole album will of course be the judge, not on his ability to compose songs, but on his current ability to sing.