This viewpoint disregards the flux of the brain in favor of a vague, static "psyche". The brain is plastic, vulnerable, and swimming in a hormonal soup that it's also responsible for regulating. It can fall into feedback loops because of this.
E.g.: stress damages the hippocampus, the hippocampus loses some capacity to mitigate stress, leading to more stress that damages the hippocampus further.
In that hippocampus example, we know that SSRI drugs can reverse this damage. On the other hand, they can also lead some patients to experience increased thoughts of suicide. An anxious/depressed mind can become a suicidal mind or a healthy mind in reaction to the same stimulus. Things can happen to healthy minds that send it to those corners as well. We can't just assume that someone who died of suicide was on a fixed track to get there or that their advice from a healthy mind lead to an unhealthy mind.
I think we can safely say that the Ford-Fulkerson algorithm is not some cursed object that we should not learn about. So clearly there's some spectrum of creations and the imprints left on them by their creators, and at one end of it, there's pretty much no imprint at all.
I don't think it is dangerous in itself to read his thinking, because everyone is coming at it from a different experience than DFW. But it would be dangerous to hold his thoughts up as some ideal to strive for, as opposed to hold his thoughts up as evidence that incredibly talented people can also be incredibly broken.
Depression tends to warp perception, not letting positive thoughs through or immediately shooting them down. Abraham Lincoln was depressed too, so much that the people surrounding him removed everything he could hurt himself with from his surroundings.
I think individuals with a sunny disposition or a well functioning, supportive social circle/family are unlikely to be dragged down by writings of depressed individuals.