Using atomic state, is this guaranteed
I'd say according to my testing (mentioned here) this is not guaranteed, and is indeed not much different than using regular state.
I haven't tested the @Field trick, but regular synchronized is not working. I'd say there is no thread safe way to update state in Hubitat App or Driver (UPDATE: I was wrong - static field does solve the problem).
Given that there's no way to avoid multiple threads in any real app, I'd also go further and say that simply there's no way to update state in Hubitat without race conditions (UPDATE: I was wrong). I'll try to test @Field trick soon, and I really hope it works.