The behavior is hidden behind the StaticExclusiveOnly experimental feature flag which enables a new @_staticExclusiveOnly attribute which basically results in the behavior you're expecting, when applied to non-copyable types like the new Atomic type.
Try declaring that in your Package.swift or adding it like -enable-experimental-feature StaticExclusiveOnly to the swift invocation when using CLI.
This is incorrectly checking the client's experimental features when determining if something should be a let or not. Right, the intent was that you cannot utter @_staticExclusiveOnly unless you have the flag on. I'll make a patch here real quick.