This particular choice of words seems to be ambiguous. This sounds as if there is something unspecified about an "atomic waiting operation", but that's not the point here.

It's the choice among the eligible ones that is unspecified.

Something like "of an unspecified one among the atomic waiting opeations that are ..."
but that loses the "at least one" property.

Maybe having a generic sentence in [atomics.wait] about how the choice during unblocking is unspecified (both the number of unblocked operations and the choice which ones is unspecified).