Answer: This is a great question, and one that arises with many cars besides Mustangs. I can’t tell you for sure if the shop owner is correct, but I can tell you for sure that the “experts” are not correct.
First, in a very general sense, no two cars of any type that were built during this time are identical. They were built in large part by human beings as opposed to robots, and minor discrepancies are not unusual. In a specific sense, there are documented differences in the details of a Boss 302 depending on the plant in which it was built.
All 1970 Boss 302s were built either in Dearborn, Michigan or Metuchen, New Jersey. Each plant addressed some minor details, such as those that you describe above in their own way. Additionally, there were minor “running changes” made on the assembly line at each plant and it’s not unusual to find differences on cars that were “early 1970 cars” as opposed to “late 1970 cars.”
I know for certain that the placement of the decals on the valve covers, and the pattern of the paint on the taillight panel varied between plants. I’m not sure about the omission of certain inspection stickers. In my mind this tends to lend credibility to the shop owner. If the shop owner has a good track record of producing show winning cars, I would defer to his expertise.