(Please keep the debbugs address in the Cc header.)

The problem in #13780 is that if you mix add-hook with Customize
settings of a hook, then (depending on load order), the Customize value
for the hook may overwrite those add-hooks.  The solution proposed is
that one shouldn't have defcustoms on the hooks one may somehow expect
that packages add stuff to.

But dired-load-hook doesn't seem like one of those hooks, and if you
use Customize to be

(load-x)

where load-x is

(defun load-x ()
  (require 'dired-x))

then surely dired-x will be loaded with dired is loaded?  (Not that I've
tried myself.)