If you think about the job this photographer has, 99% of the time he is taking photos and having to be as discrete as possible, that means he probably often has to work sans flash I'm sure. But he as to always be able to get a shot, no matter the lighting conditions, so having an f/1.2 lens as your standard working lens is probably mandatory. It's a great lens, and I doubt the results would have been meaningfully better with any other lens, even though this lens probably isn't optimized for shooting as such a small aperture.
As far as the DOF, it seems proper for such a photos, as has been mentioned the flags are a compositional element, with meaning in this photo. I think the amount they are in focus, and the amount of separation between the president and the background is just about as perfect as can be. Job well done, IMO.