Restricted free agents like Baltimore's Gaither can be traded after signing their tender offer.
But those scouts have another function, to rank the Cowboys players against those from the rest of the league.
If they decide they can get even better by trading for a new starting tackle, like Gaither or McNeil, then fair enough. But there's no reason they absolutely have to, and there's no reason they must spend a high draft pick at the position, either. Free provides them that flexibility.