Exactly. Normally the debugger or repl provides reflection capabilities to be used by the editor to support warnings, errors and hints, and restructuring tools.

Such a typo catcher would be a written (I'm thinking of a lisp or lua here) on the editor side, having the information about the variable and being able to query the namespace or locals visible or not visible. The things jonathan did was just to put this into the compiler for the thrills, but it can be easily extracted, extended and customized from the outside, in emacs, vi, eclipse, ... such as jonathan did with his perl6 debugger.