the point is, that theoretically, the person who can do this the fastest, has the greatest overall finger dexterity, and therefore has potentially the greatest piano technique(naturally this excludes the thumb and octaves, etc, which can be assessed in other ways)
IMO
i believe technique is all in the mind. i can't exactly explain it, but everytime i encounter a difficult passage, it all boils down to how i think of it, or how i approach it mentally. sometimes it'as a matter of grouping the notes/figures/etc. so that we'd have to think of less things at a time, meaning, we can go faster. the dexterity of fingers i need isn't that much, it is more of the dexterity of my brain that i need to practise on becoming faster (esp with transitions of thoughts), thus doing the repetitive exercises which makes me think, instead of mechanically repeating it... (if anything physical, maybe it's my arm so that i can shift hand positions swiftly without tensing up.)
as for prodigies, sometimes we tend to think that everything is physically automatic for them. but in reality, everything that they have play had been there in their brains as techniques ever since they were a kid, and they don't even know it's technique, and it's just advantageous for them that they're not aware of it. when they encounter a difficult passage, they would know how to deal with, simply with the mere subconscious.
dexterity is partly mental, too. how fast you move those fingers is remembering/thinking how fast switching should be with those digits. but it's mostly automatic, coz we supposedly have learned it as kids (for those who started early). for those who started late, like moi, i'm just starting to gather and form good habits of the correct movements and it's taking a little bit of time because my brain isn't in it's prime for learning. but i know i'll get to that technical level and be able to play rach 3.