I think 'kanji' should be interpreted at large. This is the Chinese writing system and inventing new characters (which happens everywhere this writing system is used) add to the whole corpus of kanji/hanzi even if some are invented or used in specific countries.

峠 has a mandarin pronunciation apparently: https://baike.baidu.com/item/%E5%B3%A0/4336929

働 exists in Chinese too, meaning "labor" 峠 does not exist in Chinese although many Chinese dictionaries (most notably the CC-CEDICT) include it

>峠 "mountain pass"

Incidentially, the backstory behind that kanji is hilarious.

The kanji is composed of the kanjis for "mountain" on the left side, and "up" and "down" on the upper right and lower right sides respectively.

You know what a mountain pass does? Go up and down a mountain.

This is how a lot of kanji are formed. For example 町 (town) is 田 (rice paddy) + 丁 (street). I guess at some point in language formation a lot of towns were primarily collections of rice paddies.