I don't know if the contract is final or not. I saw some posts talking about them getting him signed on a more favorable contract than the poster would have guessed, but I haven't read the story yet.
In any case, I'm not saying he solves all O-line problems. If he starts, we still have a depth issue, and if he is the swing tackle, we still have to find a starter at OG or RT. I understand his name wasn't written in ink all year as the starter, but like you said, he held is own the games he started and played, including in the playoffs and Super Bowl, and he has started an average of 6 games for the Pats each of the last 3 years. Considering the Cowboys already have an O-Line full of All-Pros, it's not necessary that the team have another player of that caliber to have a strong O-line.
And in any case, it was wrong for the other poster to call him a scrub. It seems his belief is either a player is a star or a scrub, without there being levels in between.