I think this video clearly shows that they recorded the Wembley concert.
(the description confirms that)
It's the perfect time for a new live album. They're still at their peak, having played their biggest solo show yet, playing Idiot and Dookie in their entirety, plus the release of Saviors. It's the kind of material that needs to be released to demonstrate the scale of this band. I was there, and the atmosphere was completely special, even in the days before the show, with so many people walking the streets wearing Green Day shirts. It felt like 2005 all over again.