American Cinematographer would be the most reliable resource for details on the cinematography and original 2K DI process. Unfortunately, I don't have a subscription with access to their article on The Departed.
I highly doubt a new scan of the OCN or any film-out was done for this release...
I’m watching it now. I suspect it’s a 4K upscale of the 2K DI with a new, richer grade. Regardless of the heritage, it looks significantly better than the Blu-ray.
I wouldn't rely on iPhone screengrabs for that determination. The screengrabs are 2778x1284, which is ~2.16:1 and, not coincidentally, the native ratio of certain iPhone screens. The HD screenshots are zoomed to fill that screen. The UHD grabs are not zoomed.