I do not think that the reading explanation is right for this vocab (see picture below). I think けん turns to げん because げん is another on’yomi reading of 拳 that is not listed in WaniKani, not because of a rendaku. Please someone correct me if I am wrong 
I’ve been wondering about that too. I don’t think it’s technically rendaku. According to jisho.org, the on’yomi of 拳 is both けん and げん, so that’s probably just a miss on Koichi’s part
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I think Koichi is clever enough to know that this isn’t technically a Rendaku. It’s just very convenient to call it that.
Maybe the dictionaries included げん because it got rendaku’d very often. Maybe a historical thing. Good catch.
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@jneapan Looks like we have the same opinion on this! 
@Heiopei I made this topic because I do not understand the reading explanation, and I am wondering if this is due to a lack of knowledge from my side. And if I’m right, then I am just reporting a small error. The purpose isn’t to judge if someone is clever or not…
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OK I’m fairly sure about this: Rendaku is never on the initial mora (syllable) of a word.
That means if you do see Dakuten on an initial mora, this is not due to Rendaku,
but because it’s a normal reading of the used Kanji.
Thus the 3 or so words that have 拳 being read げん at the beginning confirm that げん is a reading of the kanji 拳, and that these words are not cases of Rendaku.
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oh yeah. Forgot about fact of rendaku.
Alright, then we agree this is not a Rendaku, it’s just another on-yomi reading. Case closed
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Please please please email (hello@wanikani.com) or @ me when you guys find this stuff. It’s great when you all find errors and figure them out, but we can’t fix them if you don’t tell us. 
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I was reading this thread and thinking to myself “somebody email them and let them know!”
Good catch guys.
Great you noticed it @anon20839864
Duly noted