`extend self` and `module_function` are not the same

extend self and module_function are not the same. The guide says:

Favor the use of module_function over extend self when you want to turn a module's instance methods into class methods.
https://github.com/bbatsov/ruby-style-guide#module-function

Unfortunately, extend self keeps the instance methods public, but module_function makes them private.

# `extend self` makes the copies public
module Pub
  extend self
  def x; end
end

class PubClass
  include Pub
end

PubClass.new.x #=> nil

# `module_function` makes the copies private
module Priv
  def x; end
  module_function :x
end

class PrivClass
  include Priv
end

PrivClass.new.x
# NoMethodError: private method `x' called for #<PrivClass:0x007fba1c0e7e90>

Is making them private the point?

Start with the module_function section in the linked Ruby style guide and review the examples in this issue. Determine whether the guidance should explain the visibility difference or be changed; done means the section clearly and accurately describes the relationship between extend self and module_function.