They've said that 5e is written in natural language. They did not say that only some portions are written in natural language.
Well, it's pretty obvious that not all of it is "natural language". You yourself pointed out where the DME defined "Campaign" using terms specific to D&D.
Pretty sure terms like "Adventure", "Hit Points", "Experience Points", "Hit Dice", and "Level" all have meanings in D&D that are different than what "natural language" would suggest.