Hi Radu,
You’re right, this is a typo from our side. The shaft coupler on the Step 4 is named “belt tensioner”, which is wrong. (the “real” belt tensioner is a 3D printed part that you can see on the Step 3)
This shaft coupler is customized for Niryo One, as well as the torsion spring. Those are all parts that you’ll be able to order in about one month, when we officially launch Niryo One. (subscribe to the newsletter so you don’t miss anything)