And it truly is. There's something this honestly nails.
But after using it for few moments, I was able to crystallize what I perceived was "wrong" with the design: the mechanic of combining locality with spatiality means I completely lose sight of the bigger picture. I have a really bad memory and attention span, so I depend on constantly reading cues/senses of scale from my environment. As in, they need to be constantly there. (For example, the first thing I'd do if I ever used a Mac was make the scrollbars permanently visible.)
With this UI, the immediate and global focus (the only focus) is on the comment I'm reading right now. Paradoxically, the clear intuition is that this would reduce distraction and increase focus, but (at least for me) by only presenting individual details I cannot establish and maintain orientation about how big a thread is, the structure of it (a few large mega-subthreads, or subthread islands, or hundreds of upvotes but only like 15 total comments), etc.
I tend to take full advantage of scroooooling through and cherry-picking comments to read at random. I've long dreamed of solutions that provide a rigid framework that ensure I read every comment, but I've kind of accepted that such approaches just don't work in practice; they require a tremendous amount of discipline to use (almost like grammar-proofreading a book).
A UI that has me consider every single comment as globally important ("it's the only thing on the screen at this exact moment"), both a) maxes out my attention span very quickly and b) feeds me a non-representative, breadth-first view of the discussion, because I find there's a nontrivial effort associated with needing to hit the right arrow key to expand the comments: I Have No Idea How Many Subcomments Are Hiding In There nO I aM NoT GoInG iN ThErE ThAt iS ScArY. For whatever reason (???) it's physiologically easier to just keep hitting Down instead.
This is honestly novel and interesting - and because of how well-executed this is, I can say that if you were to ever to consider tackling global orientation (I'm imagining spotlight/expose type zooming here, but I strongly suspect that would be just as disorientating as not being able to vaguely internalize the bigger picture at all), I would be very interested to see.