Hi @scarrg,
I’ve just tried the Swift migrator in Xcode 8β6 and it doesn’t seem to suffer from this problem.
It has many, many errors in SwiftyJSON post-migration, but only a couple in the project code. I’d suggest waiting for Swift 3 to become GM with a later seed of Xcode 8, and then go and grab the updated version of SwiftyJSON from the project’s GitHub page.
I’ll also suggest to the macOS team lead that this tutorial is put on the list of ones to get updated once macOS Sierra is released.
sam