Looking at the patent license, it applies "solely to permit the reading and writing of image files that comply with the DNG Specification". So if you try to create a derivative/modified format based on DNG for whatever reason, the patent license won't apply since the new format would not comply with the specification. (Though that doesn't necessarily mean there's actually a patent threat - hard to say without research what actual patents Adobe owns that apply to the specification, and when they expire or expired.) Also, the specification document itself is not under an open copyright license, so the spec for the modified format would have to be written from scratch (though it could refer to the original).

Not sure if there is much reason to fork the DNG spec in practice, but still the situation is fairly different from "open sourcing API documentation with an Apache license".