exists according to a whole bunch of other Japanese sources
just try googling the word
e.g.
https://www.weblio.jp/content/%E8%A9%B1%E9%A0%AD
https://ja.wiktionary.org/wiki/%E8%A9%B1%E9%A0%AD
Doesn't Kenkyusha primarily publish English-Japanese or Japanese-English dictionaries? They're not even trying to make a comprehensive dictionary for Japanese. There are going to be tons of words that aren't covered but certainly exist.
Plus, just generally, it's not strange for real words not to be in dictionaries. Dictionaries are updated slower than actual language evolves.
Dictionaries tell you about words that exist, they don't tell you which words don't exist. I have six dictionaries here that list that exact word. That being said, the Green Goddess is very extensive. Indeed rare to come across something that it doesn't list (and I checked, it's indeed not included).
わとう 話頭
The word does not exist according to the kenkyusha.