If the entire field was sitting at +3 or +4, then yea you might have an argument. Watson is having the tournament of his life. What it shows more than anything is that a good golfer past his prime can still play with the best despite not hitting it as far is he has a great week, plays from the fairway, and putts well.
Also, how do we know that Phil Mickeslon, Ernie Els, Vijay Singh, Retief Goosen, or Jim Furyk won't come out of nowhere and have a spectacular couple days to be in contention in a major in their late 50s?