Its corpus is existing songs but I think it's more like it generates lines based on the previous lines while aiming to stick to the subject matter and optimize rhyme density.
I assumed that too, but all of the lines seem to be 1:1 copies.
I skimmed through the paper. All of the lines are 1:1 copies, it does not generate its own 'unique' lines. Instead you provide it a keyword (in this case love) and it will find phrases with the word 'love' (in the database of lyrics) in it that rhyme with the previous line.