"given the relative abundance of cash available to EA orgs (through OpenPhil and Good Ventures), a rate as high as this is surprising."

Three thoughts: i) the range of figures offered was very wide, ii) orgs that are strongly supported by large institutional donors gave lower figures, iii) if those orgs expect OpenPhil/GoodVentures/etc to give them even more money in future years, they could still sensibly report a high discount rate.