I was also hoping to see Mantle on Civ: Beyond Earth for Linux... but it doesn't seem to be the case :???:
http://www.phoronix.com/scan.php?page=news_item&px=MTY2NDA
Also the PCGH's vid was running on a PS4 Devkit:
http://www.dsogaming.com/news/report-latest-pc-trailer-from-watch_dogs-is-actually-from-the-ps4-build/