Thanks a lot for your feedback.
Indeed, the release only one tether is not used in the game, it is what remains of an idea where there were 3 anchors and you had to slingshot yourself by removing one tether to fly between the other two then release the remaining two. It didn't make the cut for the release because it was too janky.
I guess if I had to do it again, I'd put right mouse button to remove the tether and make it possible to hold the button to have it grapple when the anchor is in view.