I believe is correct. By making a fork of the Unreal Engine source code and adding in your modding tools, you can make that fork available to modders, but they will only be able to access it if they have an Unreal Engine account linked to their GitHub account (which is the intent of the EULA language). Unfortunately I am not a lawyer, so if you need a definitive answer to your question it may be best to ask on the AnswerHub. Our legal team catches most of the questions that are posted in the Legal & Licensing section.