I think it's a bit of choking and a lot of Serena playing better. I remember ONE point where Mauresmo really choked. On one break point in the marathon game, she hit an easy forehand (maybe backhand) wide. This would've changed the match and she knows it. When she saw the opportunity to finish the point and the game right there, she completely missed her shot... On other opportunities, Serena may have lifted her game, but Mauresmo choked on many easy points... So, a bit of both.
You may be right about the fact that she'll never beat Williams again, but then again, what do we know about the future, right ? One thing's for sure, Amélie can't start thinking this. If she still has pride, desire to win, she won't have trouble finding the motivation. She may be crushed for a few days, but after that, it's back to work.