I assume the reference types properties in Swift are nonatomic.
I just performed a simple test which I think answers my question. I created a new Swift class inherited from NSObject and added a reference type property. In the generated header for Objective-C it shows that property declared as nonatomic.
Confirming that Swift properties are nonatomic in the ObjC sense. One reason is so you think about whether per-property atomicity is sufficient for your needs.