With the 5D mark II, ISO 1600 is the top practical ISO for many people: the images look good at 1600, especially on close-ups of one or two or three people. Yes, the DR is wider at base ISO, and the color is a bit richer too, but as long as there is not a lot of underexposure, 1600 on the 5D-II will look quite acceptable. And yes a slight bit of digital noise will look acceptable to many people, especially in smaller images. Second: he might have been "foofing", something a very famous Canon wedding shooter advocated: using the 5D-II, the famous fellow would do what he called "foofing", or using on-camera Canon speedlight, bounced up and off of surfaces, often DISTANT walls or ceilings, at ISO's of up to 6,400, and often with the lens at f/2.8. This flash is weak on the longest of throws, but it FILLS IN shadows, and that kills noise. It only takes a tiny bit of flash fill to eliminate noise where it shows up the most: in under-exposed areas.
With a High Speed Sync capable flash, the photgrapher can control backgrounds by throwing them out of focus, AND shoot at fast shutter speeds, due to the elevated ISO level, AND can use wide lens openings like f/2. f/2.8, f/3.2 and so on. This can create a beautiful, beautiful look: heavy background defocus, shallow DOF, and very subtle flash fill for sparkle, and nice light that is not overly contrasty, and the flash kills noise in the shadows. This creates a VERY easy-to-adjust raw file from a great FF pro cam like a 5D-II.
The famous wedding shooter who coined the term "foofing" was Denis Reggie, prhaps one of the most-famous wedding shooters of the last 30 years. Perhaps the wedding shooter who shot your wedding set up his camera or cameras to be at 1600 ISO, for the "worst-of" conditions at the wedding venue. Maybe he planned the enire shoot around the 35/1.4-L and the 50/1.2-L, and planned to utilize the sheer speed of such lenses. Maybe he wanted everything to be at ISO 1,600, so he would neer have to make an ISO adjustment during the event.