Eight's documentation is in a terrible state (in part because there are still many things about which I've yet to make up my mind), so blame me for any confusion.
Here's the gist: Fexprs, like macros, take expressions as arguments (duh). Those expressions are made up of symbols (duh). Because a fexpr is evaluated at runtime, those symbols may already be bound to values when the fexpr is called. Eight keeps track of which symbol is bound to which value at the place the expression originated (where the programmer wrote it) --- even if you cons expressions together, or chop them into pieces. This eliminates the need for (uniq), but still allows for anaphoric fexprs when symbol-leaking is desired.
When I wrote the docs on github, I called an expression plus any accompanying bindings a 'closure' (even though it wasn't a function). I also didn't know the word 'fexpr'. I've read a few dozen more old lisp papers since then, and hopefully on the next go-round my vocabulary will be much improved.