[AgentME]

Note that the lyrics are part of the input to the Jukebox neural net, so I assume they used the lyrics of an existing song here. Nothing stops someone from using a lyric-generating neural net with Jukebox though. (It's probably more useful that the lyrics aren't produced by Jukebox because it means you can easily swap out the lyric-generation part or manually tweak the lyrics.)

[virgil_disgr4ce]

The lyrics are generated by a separate model, but they're "co-written" (cherry-picked) by the authors.

[AgentME]

Only one category of the samples on the main Jukebox page are described that way. The rest of the samples were pre-existing lyrics, so the song linked above might also have had pre-existing lyrics.