I'd like to know what makes that ssyntax too. I've heard ssyntax called "symbol syntax"[1] for its low-level details, as well as "special syntax"[2] for, well, reasons I haven't figured out, which is why I ask. :-p
Looking back at "Being Popular" and "Arc at 3 Weeks,"[3] I wonder if pg just used "ssyntax" as an in-code abbreviation for "syntax for s-expressions," in which case that's something lark totally is exploring. But even then, just the term s-expression can mean different things to different people, and I'm curious.