I think that applies to the top runners, who are competing with a few other specific individuals, but a typical marathon runner will be competing against the clock, if anything. In a race in which no participant is paying any attention to any of the other competitors, they will each still end up running with others going at about the same speed, assuming their pace does not fluctuate erratically (which it doesn't - especially in longer races, where almost all participants have practiced enough that they have an intuitive knowledge of what they can realistically expect to achieve.)
