How often do you edit patch files?
When editing a patch file, how often do you edit metadata beyond the file content differences?
It seems that the proposed DiffX is meant to be produced/edited only by machines, so JSON doesn't seem too much of a problem for this use case.
You missed the parser part and mistakenly focused on editability.
Diffx authors wrote a prototype in Python where JSON support is built-in. Go parse it in bash or C.
Having to parse JSON for the sake of applying a patch is not exactly wise