Swan song is like the final goodbye and farewell to something or someone. For example, a great athlete in their last game, the announcer may say "here goes David Beckham in his swan song" as the clock ticks down to 0. Epilogue is at the end of a book, the story is finished and the epilogue is a few paragraphs or pages at the end, sort of like in Harry Potter them saying he grew up and was happily ever after or whatever. I interpret the line as saying, welp, humans have brought about our own end, killing our host planet and ripping each other apart, let's be present for the reading of our swan song and observe and experience our demise, let's be here for our own epilogue to the human storybook