Your Achilles Tendon isn’t a loose hanging thread hanging between your knee and ankle! It doesn’t “recoil” up your leg when it’s severed!
It’s firmly attached to the muscles and whatever else. If you saw movement on the calf, it was probably a convulsion of the muscle, pain or something like that.
I actually DO know this. Mine was completely severed about 20 years ago and believe me, it stays right there in place - severed or ruptured. Had surgery and it was sewn back together and a 100% recovery. I even asked the surgeon about hearing that and they laughed and said that is never true!
How do I know this? I've ruptured both my patellar tendons, pulling them off my kneecap. When that happens, the quad muscle "windowshades" up. It is pretty important to reattach the tendon within 2 weeks of injury, otherwise the quad muscle "sets" in contraction and is much harder to stretch back to its original length.