I am not sure where you got that idea. I am sure Dan is a fine fellow. However, I am not looking for what he is offering.
And what were they (Tohei, for ex) missing when they went outside the art? They can't speak to us but we can speak for ourselves.
I know I was missing being able to defend myself in my early years. So out I went to find a way to be safe.
Now I only train in Aikido. I am not judging others as wrong I am really interested in the thoughts and process.
I think that you're confusing things if you think that internal strength is all about fighting or self-defense. Of course, the applications are obvious, but what we're talking about is really a superior way of moving and using your body.
For my money, it's not "outside" of Aikido at all - unless what Morihei Ueshiba was doing was "outside" of Aikido.
I also think that Mark's distinction between "inner strength" and "internal strength" is very important. On the other hand, I think that the two were intimately connected in Morihei Ueshiba's training method - so much so that you need one to understand the other.