“Essential Claim” means a claim of a patent, whenever and wherever issued, that Adobe has the right to license without payment of royalty or other fee that is unavoidably infringed by implementation of the DNG Specification (from https://helpx.adobe.com/photoshop/digital-negative.html#dng)

The patents are not on the file format, but cover techniques required to implement an application reading it. Similar to how you have to pay to use e.g. MPEG4, since various companies have patented parts of the compression mechanisms. The patents don't cover just MPEG4, but any video format using these mechanisms.