I think he's talking about run-time environments a la Kernel, Emacs Lisp, etc.
Nulan and Arc/Nu use a compile-time environment to replace symbols with boxes at compile-time. But that feels quite a bit different in practice from run-time environments (it's faster too).
"Are you suggesting a third thing we could insert here?"
Once again, I think he's referring to run-time environments. Basically, what he's saying is that you would use boxes at compile-time (like Nulan), but you would also have first-class environments with vau. The benefit of this system is that it's faster than a naive Kernel implementation ('cause of boxes), but you still have the full dynamicism of first-class run-time environments. I suspect there'll be all kinds of strange interactions and corner cases though.
Slightly off-topic, but... I would like to point out that if the run-time environments are immutable hash tables of boxes, you effectively create hyper-static scope, even if everything runs at run-time (no compile-time).
On the other hand, if you create the boxes at compile-time, then you can create hyper-static scope even if the hash table is mutable (the hash table in Nulan is mutable, for instance).