Domain and page authority in this case is relatively useless. The problem with lyrics SEO is rampant duplicate content. Every site has the same content and since Rap Genius is relatively new to the game, the way to climb the rankings is to do exactly what they've excelled at: getting their users to submit unique content. It's the same reason songmeanings.net has succeeded as well.

Your comment about storing each comment in a separate URL seems to be the true value here. Then each landing page can distinguish itself from the dozens of other lyric clone sites.

Disclaimer: used to work for Tunewiki, another lyric site