All top speeds need to be reality checked by doing the same run in the opposite direction.
Test --> report back.
I hear that. I don't know what my top speed is. I haven't tried in a very long time. And even when I did, it was never under any kind of "rested" or wind-free conditions.
I think the statistic means something, But I think Carmichael picked the "12-mile time trial" as a real measuring stick for evaluating differing training loads and abilities.