I'm able to access a private method in a children model class

Question

Trying to do Single Table Inheritance using Rails 5.0.0.1(latest), and if you do:

class Parent < ApplicationRecord
  private
  def self.some_method=(argument)
    return true
  end
end

class Child < Parent
end

$ rails console
irb(main):001:0> Client.some_method
=> true

The code will works, which is not what I expected, it should work only if is protected. Could anyone explain why this code is working? Thanks in advance.