As a Chinese speaker, I know what you mean: ‘Oh look! I know what this kanji might mean! Wait, what’s this word it’s in though?’
I know this isn’t universal, but I think it’s fairly common for kanji to have similar kun’yomi across words, provided those words are related. Of course, there’s no way for us to know for sure if they’re related – I hadn’t seen 暴れる or 暴く before you mentioned them – but we can still use known kanji readings to help up make guesses.
I guess WK just doesn’t want to overload people with more than one keyword per kanji? I’m gonna guess that the keyword for 暴 was ‘violence’? Some stuff just has to be learnt through exposure. (Oops, seems like ‘exposure’ is unexpectedly appropriate here, even if it’s a different sort of exposure.) Then again, I think part of why I remember this is because 爆 and 暴 sound exactly the same in Mandarin (bào), and since they both express fairly violent ideas, I treat the two of them as almost the same thing. I frankly shouldn’t, but I haven’t used Mandarin in a while, so I’m starting to mix them up. And I mean, ‘violence’ is violent, ‘explosions’ are violent… and when something is suddenly ‘exposed’ or ‘revealed’… it’s like a a bag/dress/shirt/suitcase/folder of documents bursting open. That’s ‘violent’ too!
I think this is really just because it’s a rather technical word, and the kanji is (I think) fairly rarely used? All three forms (如く, 如き, 如し) seem to be in use, but only in fairly formal/archaic contexts, and each has to be used differently: the first as an adverb; the second as an adjective coming before nouns or before は, in which case it’s like a noun; and the last as a sentence-final adjective that probably only appears in fairly old literature or in sentences meant to sound like Old Japanese (e.g. proverbs). That’s probably why they don’t turn up on WK. I could be wrong, but I doubt that WK wants to tackle words that have a tendency to only appear in supposedly ‘high-level’ grammar points, like 余儀, which appears in the structure 〜を余儀なくされる, which tends to be labelled as ‘N1’. Vocabulary is one thing, but teaching words that carrying grammatical baggage is probably less appealing since WK users can’t employ them without first learning how they should be used. (I’m just making guesses here though, since I don’t use WK myself. I’m just on the forums.)