ARC is implemented at the language level but it's not required. You can use it when and where you like.
One fundamental aspect of swift is the distinction between reference types -- which are reference counted -- and value types -- which are not. Generally in Swift you'd use a value type over a reference type unless you have reasons not to. E.g.: https://developer.apple.com/documentation/swift/choosing_bet...
I mean, I don't know what the right approach for this library is. The authors are going to have to fix their own code. IMO, coming up with a demonstrably poor solution and trying to defend it as "idiomatic" is pretty weak.