It seems like properly enabling lexical binding in ediff will be a bit
tricky.

If you look at the code of ediff-find-file, you will see the use of
`symbol-value' everywhere.

This line is the source of the error in the bug report:

(defun ediff-find-file (file-var buffer-name &optional last-dir hooks-var)
  "Visit FILE and arrange its buffer to Ediff's liking.
...

deleted."
  (let* ((file (symbol-value file-var))  ;<------------------

I see symbol-value used at 4 places in ediff.el. So all of those will need
replaced with variable access in lexical scope.

--
Kaushal Modi