Please just describe what they are doing and how it is different than Aikido and contains what they claim is lacking even in original students of O'Sensei.
Dan and other "internal" teachers are not teaching aikido. It is about enhancing your body. They teach how to rewire your body and mind by specific solo and paired exercises. When done right, it takes time and a lot of work before for these exercises have noticable effect. And it takes more time and more work before these exercises have a profound effect. It is not about becoming muscular, hard and tough. It is about becoming extremely centered, solid, soft, flexible and sensitive (in my understanding).
These exercises are not waza, they are more like calisthenics (but very different from the normal calisthenics. I'd say it's a bit like yoga in motion). In that light it might not be a surprise that the exercises are cooperative. Your partner offers just enough resistance that you can notice your mistakes, fix them and learn).
Sometimes the teacher might demonstrate how we could apply these internal skills in a martial context. But it is not a martial art by itself. It does not replace aikido, karate or BJJ training. It is something you do on your own, besides the art that you train.
Why would an aikidoka want to add internal qualities to his body? Points that Dan and others are trying to make, is that aikido (like many martial arts) was supposed to have these internal qualities from the beginning. That aikido is not unique in this. That O Sensei spent a lot of time doing solo exercises like these. That O Sensei tried to teach it. That somehow, it didn't get transmitted all too well. All the references to the history of aikido and non-aikido martial traditions like taiji is meant to back this up.
Again, I'm no expert in history. It is just my understanding of the intention of Dan and others who promote internal training for aikido.
I am not interested in history that much, but they have convinced me that aikido is supposed to be an internal art. That's enough for me to keep doing these exercises on my own.