module_function is not the same as self.method

module_function.rb

# This works
module TestOne
  def module_method
    puts "TestOne works"
  end

  module_function :module_method

  class A
    include TestOne
    def instance_method
      module_method
    end
  end
end

# This fails
module TestTwo
  def self.module_method
    puts "TestTwo fails"
  end

  class A
    include TestTwo
    def instance_method
      module_method
    end
  end
end

TestOne::A.new.instance_method
TestTwo::A.new.instance_method

# Output:
# 
# TestOne works
# module_function.rb:25:in `instance_method': undefined local variable or method `module_method' for #<TestTwo::A:0x0000010084d400> (NameError)
#        from module_function.rb:31:in `<main>'