I assume it is mostly because it is the "Peaceful Pill" and not the "Peaceful Suicide" Handbook. It simply focus on using toxic substances to poison yourself (in a similar way to "official" euthanasia) instead of talking about any other ways to CTB, since there are a lot and they are probably harder to compare. Not sure if they have other reasons.
I wouldn't think too much about it, you can research about guns and hanging in other places.