(Consider that the mental aspect of a race is inversely correlated to the distance of a race, and that muscle glycogen is generally depleted around 2 hours into a race-pace bout)
I disagree with your quoted statement. I'd say the mental aspect is equally important in the 1/2 marathon as it is in the 5k. In the 1/2s I ran, the mental part was mostly fighting fatigue, whereas in the 5k, it was mostly fighting through pain (and fatigue too, but anaerobic fatigue instead of aerobic)
However, for certain people who might be described as having "no business" at a race, the longer races are going to lean more heavily on a will to complete the distance than an ability to maximize performance. I would guess that for some people (certainly me when I was starting out) their 400m pace may not be that far off from their marathon pace.