We don't have a video of him recording Down as far as I know, we just have a video of him writing it. The author of the tab book is very specific about which guitar parts in which songs are in different tunings, which makes me guess that he had some studio notes telling him what tuning the different parts are in. Otherwise he wouldn't have have bothered transcribing Down in 1.5 down, he would have just done it in standard tuning like all of us. That's my analysis anyways.
i have the tab book, it's so fucking wrong. you're better off using the internet, or working it out for yourself.