The Four Programmers Sketch (Four old programmers sit reminiscing about the "good old days" of coding.)
Programmer 1: Aye, kids these days have it easy. Writing in Python, JavaScript—high-level languages that do everything for ‘em. Back in my day, we had to write in C!
Programmer 2: C? You were lucky! We had to code in assembly—punching in opcodes by hand!
Programmer 3: Assembly? Luxury! We only had ones and zeroes, flipping switches on the front panel to program each instruction!
Programmer 4: Ones and zeroes? Pfft. We only had zeroes! And we were grateful for ‘em!
You programmed with a keyboard, luxury! We manipulated the bits on a breadboard.
The syntax of some of these examples seems more verbose and harder to follow than just writing the XML or JSON itself, especially the anchor examples. Aside from that, XML to JSON conversion exists, though with limitations, seemingly the same ones, such as no comments in JSON so instead you're adding an additional key. Might as well just write one or the other and convert, especially because libxmq is nearly JSON anyway.
This seems to be an effort in part at minification which in JSON can be achieved by other efforts such as shortening the keys and having an agreed map on both sides. If they're going for readability, I'm not sure if this is more readable than JSON. It might appear more readable than XML. I look forward to this standard becoming the 15th universal standard.
The discussion forum on GitHub is inactive at the moment with 1 discussion about windows. As for adding in comments to JSON that's probably not required if both parties understand the payload being exchanged and as you mentioned can easily be worked around by adding in a "_comments" key/value pair.
As I read it with the style chosen, it seems to be more like JavaScript or Java (given the Authors background). This seems reads like someone who wants the features of XML added into JSON, but didn't want to call it JSONX. Having just googled this, the name is already taken by IBM for JSON as XML.
"Aww, if you make me cry anymore, you'll fog up my helmet." -- "Visionaries" cartoon