It seems to have been left over from a past change:

* lisp/autorevert.el: Use lexical-binding.  Fix hook usage.
  2015-05-22 00:03:12 -0400
  https://git.savannah.gnu.org/cgit/emacs.git/commit/?id=9e41e0bc6a6fdb7a01841db62d8de42d67be534a

> Could you pls check whether the patch works for you with magit? (I
> don't use magit myself)

Works for me; I hope to see it land soon.  See also my other message for
a related but more subtle problem: https://debbugs.gnu.org/34847#16

Thanks,

-- 
Basil