It was preserved in the sense a fossil is preserved. The object was destroyed but some of it's structure remains. (Which is ludicrously cool. The world is so strange and wonderful. Though it's also a world where a volcano can erupt in your vicinity.)
Also, it's a great lesson in randomness - that day when that mountain erupted, the wind was blowing away from Naples, essentially sparing the disaster for its residents. Imagine if it was the other way around?
Not a volcanologist, and vastly simplified, but...
After the collapse of the eruption column, when it's roaring down the volcano's flanks and incinerating/burying everything in its path, it is driven by the local topography.
But before that collapse - especially toward the end, when it's getting seriously unstable - the wind can have huge effects on the direction in which it "fall down".
And the wind (at altitude) pretty much controls which way the ash fall goes. That's technically not pyroclastic flows - but often just as deadly for a major eruption.
- And note that descriptive terms for volcanic eruptions (Plinean, Pelean, Vesuvian, etc.) are subject to "it's all constantly-changing shades of gray" caveats.
The wind _positions_ the ash cloud and then the terrain channels it once it falls. I suppose I'd been thinking of a Mount St Helens style flow where the mountain gave way.
I don't think that phrasing implying there was a whole brain instead of a part fares any better by tying it to a definition of fossil. It's not essential to the definition of fossil that what is recovered is a smaller subset of the whole thing. A fossil can be a fossil of an entire organism.
I think the that commenter's point is that the article makes numerous unqualified references to "the brain" which on a most natural reading would imply the whole brain when it's some (albeit very interesting) individual shards.
