The issue with using that Nightwish track from LBP2 is that Sony need to go back to Nightwish's label and get the rights again. It would have been a one-time only deal, I imagine. Superbot may not have the resources to go chasing after this. Even the actual music used in the boss fight against the Negativatron (The Gadfly Suite: Finale) is apparently a film score, so copyright issues get in the way there too (although it's a Russian film from the 50s, so not sure if there's still a copyright on it...)
They'd be more likely to use the actual "The Cosmos" music used in the levels leading up to the final boss, since that's original music composed for LBP2, and therefore Sony own the rights to it, and would still fit the level.
This music, specifically: