I don't find their closing comment right however:
“What I conclude from this, as apparently Tegmark and Bostrom do as well, is that human activities, including accelerator science, contribute a tiny amount to the extremely low probability of doomsday catastrophes from any source in our neighborhood.”
With the onset of uncontrolled environmental changes, global warming, arms proliferation, nuclear proliferation, bio-genetics, nano-works etc. I think it is safe that we are at the same time our largest threat. I don't think it can be labelled "Tiny". Then it depends on what you understand by doomsday. While it is improbable we will end up under grey goo scenarios, it cannot be put aside that this could significantly alter the quality of living and create large scale devastation centered upon human intervention.
A nice bright naked eye comet would be about right at the moment... oh well...
The impossible often has a kind of integrity the merely improbable lacks. -Douglas Adams