Do not feel bad, you are not the first aikido person who thinks they're relaxed and aren't. I think it's safe to say that every aikido person who starts working on these things thinks that and quickly learns that we weren't anywhere near as relaxed as we thought we were.
I totally agree with that, it's just that what you are describing sounds like a process I already have been going through (changing from muscle to transmission and manipulation of extrinsic forces like grf and weight). MAN I was stiff 2 years ago! (have been all my life)
But yeah I saw the same thing you did about the biceps firing. I'll keep watching that video but I still have not convinced myself that the power I am primarily using comes from those muscles. (They may be firing here and there, yet still be inconsequential to my main forces and movement-- there is a lot of noise in my motor system, I need to do more misogi ;])
Do you have a video showing your relaxed body causing fast or instant kuzushi?