If he actually donates the money then the charity is not constrained to fantasy. By the miracle of the world banking network, people thousands of literal miles away can be brought as close as the sphere of action. Those concentric rings are measured in frequency and impactfulness of interaction, not physical distance.

What Screwtape is advocating is that he simply intend to donate the money once Givewell publishes a truely definitive report (which they never will). Or better, that he feel great compassion for people so many steps removed that he could not possibly do anything for them (perhaps the people of North Korea, who are beyond the reach of most charities due to government interdiction).