Ok, I see that the first snapshot of Swift 5.1 was published on July, 10 and there are no releases yet.
Right in the beginning this article says Swift 5.1 is finally out! which means that it is finally released which is not true.
Some things from that article are still in review on swift forums, so it is too early to write that article IMO.
Swift 5.1 is in Xcode 11. As we have done every year for the past many years, we have moved all of our tutorial development onto the new Xcode. We’re no longer developing content for Xcode 10.
I’m finding the compatibility of Swift 5.1 a bit confusing. Even Apple’s own documentation is not clear. Some language features seem to be available for any iOS version, but others are tied to iOS 13. For example BidirectionalCollection.difference(from:) only works with iOS13 whereas Self for static members works for any version of iOS.
I think your test in diffing ordered collections should be for IOS version, not Swift language version. i.e. if #available(iOS 13, *)