I got Ao to a working state a few months before that exchange on the guile-users mailing list.
Guile works fine for Ao – it's clean and simple, plays nice with C FFI, and has a good REPL.
Racket was lower-impedance for the project discussed in that thread (and certainly has better documentation), but seems a bit heavy-weight for embedded use.
Racket always seemed a bit heavyweight to me in general. I'm more of a CHICKEN fan myself, though. You clearly are on the other side of that particular fight (r6rs vs. r4/5/7rs) though. Which is fine.
https://news.ycombinator.com/item?id=12033059
I don't get it, has he given up on Guile or not:
http://lists.gnu.org/archive/html/guile-user/2016-06/msg0002...
http://lists.gnu.org/archive/html/guile-user/2016-06/msg0002...