whereas in Anarki it goes through the intermediate make-br-fn. What's wrong with the direct expansion? I tried searching this forum for an explanation but couldn't find one.
Probably so that it can be changed more easily (by redefining 'make-br-fn rather than hacking the reader).
;arc.arc from Anarki
(mac make-br-fn (body) `(fn (_) ,body))
I recall at one point people talking about making brackets work so that, for example, [+ _x _y _z] = (fn (_x _y _z) (+ _x _y _z)). It would extract all arguments from the body beginning with _, alphabetize them, and insert them into the argument list. I don't know if Anarki actually did this, but I do believe it accepted [+ _1 _2 _3 ...] at one point.
I implemented that feature (multiple automatic parameters, with alphabetic ordering, so [cons _y _x] = (fn (_x _y) (cons _y _x))) in anarki. I don't know for sure, but I would assume it's still there.
Nitpicking: that's not ssexpansion. Ssexpansion is short for symbol-syntax expansion, in which the occurrence of special characters in symbols tells the arc compiler to transform them, things like foo.bar -> (foo bar), foo!bar -> (foo 'bar). The brace transformation is actually accomplished via changing the reader table of the underlying scheme, and so is translated at read-time rather than compile-time.
This is important if, eg, you're writing scheme code to integrate into the arc runtime in some fashion; normally, in plt scheme, braces [] and parentheses () are interchangeable; but arc changes the way [] is parsed, so you basically have to avoid using them in scheme code. Whereas ssyntax is parsed by the arc compiler, not the reader, so you don't need to worry about it.
