Earlier quoted context omitted.
That letter is not about "superhuman machine intelligence", it is about "ensuring that AI remains robust and beneficial." Among the threats cited are: autonomous vehicles, machines that need to make ethical decisions, autonomous weapons, surveillance, and issues of verification, validity, and control. Words like "singularity", "superhuman", and "AGI" do not appear.
Superintelligence and intelligence explosion are mentioned as valuable things to investigate.
Finally, research on the possibility of superintelligent machines or rapid, sustained self-improvement (“intelligence explosion”) has been highlighted by past and current projects on the future of AI as potentially valuable to the project of maintaining reliable control in the long term. The the AAAI 2008–09 Presidential Panel on Long-Term AI Futures’ “Subgroup on Pace, Concerns, and Control” stated that:
"There was overall skepticism about the prospect of an intelligence explosion... Nevertheless, there was a shared sense that additional research would be valuable on methods for understanding and verifying the range of behaviors of complex computational systems to minimize unexpected outcomes. Some panelists recommended that more research needs to be done to better define “intelligence explosion,” and also to better formulate different classes of such accelerating intelligences. Technical work would likely lead to enhanced understanding of the likelihood of such phenomena, and the nature, risks, and overall outcomes associated with different conceived variants."
Stanford’s One-Hundred Year Study of Artificial Intelligence includes “Loss of Control of AI systems” as an area of study, specifically highlighting concerns over the possibility that:
"we could one day lose control of AI systems via the rise of superintelligences that do not act in accordance with human wishes – and that such powerful systems would threaten humanity. Are such dystopic outcomes possible? If so, how might these situations arise? ...What kind of investments in research should be made to better understand and to address the possibility of the rise of a dangerous superintelligence or the occurrence of an 'intelligence explosion'?"
Research in this area could include any of the long-term research priorities listed above, as well as theoretical and forecasting work on intelligence explosion and superintelligence, and could extend or critique existing approaches begun by groups such as the Machine Intelligence Research Institute.