[nmca]

You misunderstand critically that this is not "singing along", it's generating the music and voice. Conditioning on lyrics is optional, and done "unaligned", eg by arbitrarily encoding the lyrics and passing them as additional input.

[aasasd]

Indeed, the extent of generation is obvious in the ‘continuation’ mode on any track that is rather familiar for the listener (ahem Rick Astley). Besides, in the full sample browser there are tracks without lyrics.