That's true. I don't want to criticize Guile or Lisp implementations in general, which have a number of features that Lua can't easily emulate (a numeric tower, true continuations, probably a whole lot more I'm not aware of), but I think Guile and Lua have very similar uses as embedded scripting languages (small, and with fast incremental compilation). Chances are if you using guile you may have considered Lua.

As for use-cases for Fnl, I would say it's more applicable for places where one would use Lua or Guile, rather than something like SBCL or any really fast, compiled Lisp.

Lua makes a pretty good dynamic language target, though, because of the many great implementations and small language spec.