My point isn't that these things are easy to learn because their presentations are so brief; personally, dependent typing took me a long time for me to wrap my head around. What I think the briefness of the operational semantics in combination with the examples of things people have built within them suggest is that they're very versatile. Even if only people working on language level features bother to learn them, the utility of their creations (all without needing to move to another language or introduce possibly incompatible extensions) is a huge asset.
It looks like you're criticizing the GP's praise of Idris' concise formal definition of its unsugared semantics. I wouldn't use such a formal definition for educating novices (like I think you're implying in your next sentence); I'd use it professionally to show that many desirable things--effect tracking, compile-time checking, and design-by-contract--derive from the same thing. Ken Iverson in his notes on mathematical notation says that one criterion for a good notation is "suggestibility"--the notation should suggest that other problems similar to those you found just now could be solved as well.
Whether formality is useful for novices, it is certainly useful for experts.