You make the same assumption yourself, every time you trade. The entire purpose of high frequency trading is to beat others to good deals -- not to do a better job of discovering good deals.
(so speed is important to you, just as water is important to a fish; it may not be what you focus on, but that doesn't mean that it's not critical).
There are market participants who make their money on pure latency arbitrage; it is a 100% speed game (because anyone can figure out that 1 - 1 = 0).
The majority of HFT falls into the "you have to be fast enough to not get run over" bucket.
I'm fairly sure this is the answer: They don't -- instead, they're just good at noticing, very quickly, the actions of the market actors who do. Which is just another way of saying that they're about beating others to the deal.
(I'd also guess that they're probably also quite good at handing recreational day-trader's asses to them.)
If you look at markets through an information-centric lens, it seems possible to draw a clear distinction between actions that introduce external data and those that are pure acts of deduction, no matter how brilliant.