The Quickest Way to Demotivate a Team

I recently saw a team of people attack a particularly challenging task. They were working on it fine, stumbling a bit, but making great progress. It was the kind of task that took a bit of trial and error.

The project leader was off working on another task, but happened to see this team working together. Unfortunately, what he noticed was that they were stumbling. He didn't notice the engaged attitudes, the teamwork or the progress they were making.

He did know how to do the task better than they did. He injected himself in the task with them, taking the central role. They disengaged. The nature of the task required some of them to stand by in minor support roles and watch. Two of them quietly stopped helping and left. The task finished successfully. Was the leader successful?