Ojoj, you lecture but don't know how a movable belay looks, better not to comment next time. I assume he made the belay around a rock horn or big block, it can also be done if there's a good tree, which is totally correct. And no, THIS ISN'T a movable center. What is wrong, you don't write. Except for the nut, you got it right. Instead of a set there should be a sling with a nut, then the clove hitch is a totally legit knot especially for something short to save. Mistakes though I don't see well, that the belayer is not tied to the rope and clipped with a sling to the belay center, even bigger mistake that he's 3m away from the belay?? How would he e.g. do a clove hitch arrest in case of a fall?
And I don't know what's the point in pushing up this scree and rubble, this is a "winter route".