Holden is comparing SI to other giving opportunities, not just to giving opportunities that may reduce x-risk. That's not a part of the discussion Eliezer feels he should contribute to, though. I tried to address it in the first two sections of my post above, and then in part 3 I talked about why both FHI and SI contribute unique and important value to the x-risk reduction front.

In other words: I tried to explain that for many people, x-risk is Super Duper Important, and so for those people, what matters is which charities among those reducing x-risk they should support. And then I went on to talk about SI's value for x-risk reduction in particular.

Much of the debate over x-risk as a giving opportunity in general has to do with Holden's earlier posts about expected value estimates, and SI's post on that subject (written by Steven Kaas) is still under development.