Proposed resolution:

Add a sentence to [thread.thread.id]/p1:

An object of type thread::id provides a unique identifier for each thread of execution and a single distinct value for all thread objects that do not represent a thread of execution ([thread.threads.class]). Each thread of execution has a thread::id that is not equal to the thread::id of other threads of execution and that is not equal to the thread::id of std::thread objects that do not represent threads of execution. The library may reuse the value of a thread::id of a terminated thread that can no longer be joined.