Wow, Justin, you got a lot of nerve to talk down to people like Dan Harden when you yourself don't even have one year of aikido experience. I see now why you've been so secretive about your martial credentials.
As mentioned, you are able to find it on other threads. It is just irrelevant to this thread, so it is hardly secretive.
If someone (Dan) claims that they cannot be pushed over, that is a claim that transcends aikido.
But let's get back on track. Can 'internal strength' be in the listed activities, and if so, are there and videos of it being done?
I see things like the quote from the fencing book, and fencing specific terms aside, it sounds like it could have came right out of internal martial arts books. It is 'external', and moreover, it is 'Western'.