I think you are better off setting something like $MYZSHRC and using that.

What does ${=EDITOR} do? Asking about the equals sign.

EDIT:

       ${=spec}
              Perform word splitting using the rules for SH_WORD_SPLIT during
              the evaluation of spec, but regardless of whether the parameter
              appears in double quotes; if the `=' is doubled, turn it off.
              This forces parameter expansions to be split into separate words
              before substitution, using IFS as a delimiter.  This is done by
              default in most other shells.

              Note that splitting is applied to word in the assignment forms
              of spec before the assignment to name is performed.  This
              affects the result of array assignments with the A flag.

You should just use $EDITOR there instead. No need to split words.