Good question, and TBH... I might be a bit unclear too.

Pretty-printed JSON shows siblings down, and children down and to the right. TFA's library shows siblings down and children to the right, plus with squiggly lines and boxes. So I'm not sure how TFA visualizes JSON more than JSON itself already visualizes JSON. It says it's interactive, and I don't see any interaction either other than scrolling, so I may be missing something.

But, assuming that the interaction is supposed to be stuff like hiding children in order to get a higher level view of the data, then drill down to the lower levels of the data, and provide other tools for exploring the data (searching, filtering, etc) then jless does all that too.