It's normal. If you think about how close all those components are it makes sense that they'll freewheel along with the input shaft rotation.
The older Benz transmissions would actually move the car a little in neutral if you revved it up.
Yeah, the friction & fluid coupling isn't super surprising. But I'm surprised that it spins at basically the same speed as it does when it's in D, and not easily stopped by hand*. I'm not convinced what *I* have going on right now is normal, but I also didn't do anything that would have caused it to change behavior. I'm hopefully just being extra paranoid.
* we put a wood block up to the yoke expecting to be able to easily slow it down but instead it just chewed up the wood and didn't really slow down at all.