That's right, and Rust bans that. The definition of "concurrently" typically means "without synchronization in between". An atomic access is by definition a form of synchronization.
"Relaxed atomic operations can be reordered in both directions, with respect to either ordinary memory operations or other relaxed atomic operations. But the requirement that updates must be observed in modification order disallows this if the two operations may apply to the same atomic object. (The same restriction applies to the one-way reordering of acquire/release atomic operations.)"
It guarantees that the value read was some value that was in the variable at some point. e.g. it prevents a value that toggles from 3 to 5 being read as 117. It also prevents writes from being eaten (e.g. an increment always actually occurs).
(It also guarantees happens-before for a given thread, so if a thread changes it from three to 5 and other threads are not changing it back, that thread will never read a 3 again. This also needs some level of synchronization)
> C++11 standard officially bans data races as undefined behavior.
Relaxed access of atomics is not UB in C++, that would be ridiculous. Clearly one of these two sentences is wrong or imprecise; I would bet it's the first one.
I think your initial suspicion is right. The next section: "There are several possible definitions of a data race. Probably the most intuitive definition is that it occurs when two ordinary accesses to a scalar, at least one of which is a write, are performed simultaneously by different threads. Our definition is actually quite close to this, but varies in two ways:" ... "Instead of restricting simultaneous execution, we ask that conflicting accesses by different threads be ordered by happens-before. This is equivalent in simpler cases, but the definition based on simultaneous execution would be inappropriate for weakly ordered atomics." continues at: http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2007/n248...
