I don't see that this user was ever committed to the proposal. It's harder to determine that when the proposal no longer shows up in the Current Commitments section, but a couple things indicate there was never an initial commitment:
- Instead of browsing through the entire list of users, you can total of the user's reputation across all sites and sort the committers by user reputation. Then just find the place in the list where that user should appear based on their reputation and look from there for a couple pages. This user doesn't appear anywhere.
- Just to be sure, I looked at the history of the user (mod tool). Specifically, we record an event "User is emailed 'Commitment email'" when the user followed a proposal that moves to commitment, as well as follow-up "Beta email" and "Public beta email" notifications (and some nag emails). None of those emails are recorded for the user, suggesting they were never committed to the proposal. At least one of those would have fired off at some point if they were.
While they were active and did post an answer on Area 51 Discussions around the time you mention, they never committed to the proposal itself.
To the second part of your question: users who are active during the private beta do not need to make 10 points to fulfill their commitment. I looked this up, and it's actually a lot simpler than one could imagine.
The code checks that you get 10 points to fulfill your commitment. Each question and answer gives you 1 point. Receiving the Beta badge gives you an extra 3 points. So really, users who join in the private beta need only make 7 posts to fulfill their commitment so long as they meet the other criteria for that badge, while users who join in the public beta need to make the full 10 posts.