I wondered the same thing, but notice that they ask how much you donated to them / through them. So even if we flood in and produce hundreds of responses, as long as we're honest, the analysis will show that the people who are actually donating money look like X and everyone else looks like Y, and Givewell can decide appropriately. (And if LWers really are donating a ton to Givewell or its top charities, well, then the flooding is not a problem at all.)