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'.
I am curious because you actually seem interested in developing internal strength, you do tai chi right? But you are so skeptical that when someone gives you the answers you look for you ignore them and focus on something else.
If you want to learn internal skills get off your computer and go find them, then yes than can be used in anything you like. Kevin posted at the start of this thread about his experience going from karate to aikido to bjj. His training was becoming more and more internally focused but (i would argue) that his training was getting more and more dynamic, using the body in different ways that he never had before. So I am sure he can pick up a tennis raquet and use the skills he has learnt to play tennis, just as some great internal master could use his skills to play tennis. Doesn't make either of them good at tennis though.
Now my point is any video will be useless at showing internal skills in sport because they will look like anyone else, maybe a little more relaxed or focused or centered or whatever but that won't really help you now will it.