I wonder if the drugs/poisons included SN because that is a very low rate
By definition, I would say yes, they did. However, SN became popular only recently, and these studies are a few years old. I don't have anything to back this up, but SN attempts are still probably just a small portion of all self-poisoning attempts. Just because SN has a higher success rate compared to other poisons, the number of SN attempts might still not be enough to move the needle significantly in these statistics.
Also, SN might not be as effective as you think. Even here, a lot of people fail with it. SN can be successful if someone follows the standard, recommended protocol. However, not everyone can do that. It requires getting antiemetics, which is not easy. It also requires careful planning, ensuring the person is not found and saved, ensuring they don't back out and call an ambulance themselves, etc. Planning and executing a successful attempt is not easy.
These two effects combined might explain why SN attempts can be included in the statistics, while the overall success rate of self-poisoning attempts remains so low.