That’s actually a somewhat dangerous rule-of-thumb because it is entirely possible to create compound words by just stringing non-jukugo words together, usually with something like の in between but it’s not always necessary.
It’s important to distinguish between jukugo words, which consist of two or more kanji and no kana and are for the most part Chinese loanwords, and phrases like “世の中” which is a compound (albeit not a closed compound), does consist of multiple kanji, but is not a jukugo word and does not use any on readings.
And even with actual jukugo words, it is not nearly guaranteed that they will use on readings. Some low-level exceptions are 人々, 早口 or 月見.
This is not the first time we’ve gone through this whole mess, see also: How to tell if a Jukugo word uses kun'yomi instead of on'yomi? - #8 by miss_cataclysm
It’s important to keep in mind that any rule of thumb on when to use which reading is only a rule of thumb, it cannot replace actual effort to memorize individual words and if you rely too much on your rule of thumb, it’ll only mean you’re building up a bad habit which you’ll need to unlearn.
That said, Wanikani does teach you what reading(s) to use for any given word which makes the whole thing easier than it sounds (even if it still is really annoying at times).