I'm sure you know this but IMAX film cameras weigh more than 100 kg, are extremely loud and can film only upto 3 minutes of footage, which have to be perfectly directed. There are films shot entirely shot in IMAX yes, but they are IMAX digital, such as Avengers Infinity War. Dunkirk is the only film out there shot in 70 mm film and IMAX film on a scale like that, which gives crecedence to it being equivalent to 18k(the IMAX portions at least). As for source well I can't find one for something that's non-existant. There are stuff shot entirely in IMAX film but they are all short films and nothing on the scale of Dunkirk. Shooting in IMAX film is also extremely expensive and due to the loudness of the camera, it's mostly used for action-heavy scenes. Since Dunkirk is light on dialogue and is mostly visual, it features a higher percentage of footages shot in IMAX film. You can find one if you want, I'd be happy to be proven wrong.
In recent years, there's perhaps 1917 which is an achievement in cinematography and editing, but it was shot in digital and has a laundry list of visual effects, Force Awakens used a lot of 35 mm film and Avengers as I said was shot in IMAX digital. Again I was merely quoting Nolan on that, and Dunkirk is a technical achievement, no film is out there right now on that scale. Not many directors can use it correctly, otherwise you end up with an idiot like Michael Bay who switched aspect ratio as he wanted in an engregious way in Transformers: The Last Knight.