I don't doubt that some- even many bowls-full have been smoked down to the vaunted white ash. It's happened once for me, completely by accident. I was blissfully smoking while mowing my yard, a bulk blend I had been gifted- something with strong baking chocolate/coffee overtones, when, as is a frequent occurrence for me, my pipe went out. Thinking little of it, I relit. It went right back out. A little prodding with my pipe nail revealed that I had smoked the bowl-full down to the proverbial white (it was actually pretty grey) ash.
I agree with Cosmic, that the actual event in question speaks more to the tobacco's quality that the pipeman's.
I also think the phrase is used figuratively. How often do you see a review that was "smoked to a fine white ash" be of a tobacco that the reviewer didn't like? I haven't done extensive research, but I can't off the top of my head remember any that the reviewer disliked but still smoked to a "fine white ash". Therefore, the phrase gets used as an exclamation point to an already enjoyable smoking experience, whether or not the ash was absolutely dottle-free or white/grey/charcoal/jet-black.
The way I see it, it's a colorful turn of phrase with multiple interpretations that people feel adds to their review, so it's tempting to overuse it, and with liberal poetic license.