Are there requirements for article submissions in terms of credential proof? How would that be verified?
The Editorial Guidelines can be found here. It’s not so much that you need verified credentials, but it certainly helps to be more than “Johnny Average Gym Bro with an Opinion to Share”.
Not all coaches who have articles posted are working with world class athletes or record-breakers, but I can’t think of any that aren’t at least relatively experienced and qualified trainers. Getting in touch with the editing team directly (Articles@T-Nation.com) will get you more info.