I also have diagnosed social anxiety disorder and have been in treatment for about three years, but I have a different take on the Marshawn situation. Here's my story: my situation is based around a permanent speech disorder, so it's not a matter of feeling anxiety when I meet strangers, have to give a presentation, or go on job interviews; I have fairly severe anxiety in any setting when I have to talk to anyone, family and friends included. Every word causes anxiety even though my speech disorder isn't often recognizable to others. In any public situation (e.g. grocery shopping) I have an overwhelming sense that everyone around me is judging me. After any interaction with other people - on the phone, in public, with my family, dating, sending emails, at work etc - I obsessively replay the encounter in my head and fixate on what I perceive to be negative events. These symptoms used to literally occur all day, every day, it was like an infinite loop of failure in my mind. Up until about two years ago, I would still occasionally obsess over perceived embarrassing situations from when I was a small child (< 8 years old). CBT treatments have been very successful for me. It's not easy and there is no slacking off, but I am at a point that I can control my thoughts for the most part and my anxiety isn't dictating my life choices any more.
As others have mentioned public speaking is a prevalent and common fear among a majority of people, and it's important to separate a normal fear of public speaking to those that suffer from an ongoing anxiety disorder. Social anxiety disorders aren't about the discomfort when a trigger event happens, it's about that feeling of anxiety all the time, in irrational situations, that is so strong it causes negative impacts to your life, family, and job. Frankly, regardless of "tells" like leg bouncing, avoiding eye contact, etc, I don't think there's nearly enough evidence to say where Marshawn may or may not fall on that spectrum.
I don't just like Marshawn, I love him. He's my favorite player. He clearly doesn't like giving interviews and I empathize with his discomfort. I don't personally have a problem with the NFL media policy, but I also don't think the press conference he gave this morning serves anyone very well (him, us, media, or NFL). There is ample space for Marshawn to work with the league on reasonable accommodation if he does have a diagnosable condition. Whether or not he interviews or what he says isn't going to affect how much I love him as a player, but at the same time, I'm not buying the "victim" angle in all this.