Yeah the Cowboys O-line really isnt bad when at full strength. The issue the last couple years has been more on the depth side than anything. Especially with a walking injury in Tyron Smith, and Zach Martin still elite but not getting any younger makes it tougher to say that the Cowboys Oline will end the year ranked that high, but I can certainly see why preseason they are ranked like that.
If Tyler Smith makes a smooth transition to LG, if Biadasz shows improvement with a stronger player next to him, and Steel proves that he is the real deal on the right side this Oline is in very good shape. Odds are though you are looking at missing Tyron for 4-5 games at some point in the year, and Zach Martin is probably still really good, but unrealistic to expect him to be superman.
The lack of a great blocking TE hurts, and with so much unproven depth on this Oline it makes it tough to believe in. If this unit can stay healthy most of the year then yeah a top 5 unit isnt out of the question in my mind.