The strategy-stealing assumption is "a good enough approximation that we can basically act as if it’s true". That is, for any strategy an unaligned AI could use to influence the long-run future, there is an analogous strategy that a similarly-sized group of humans can use in order to capture a similar amount of flexible influence over the future. By “flexible” is meant that humans can decide later what to do with that influence (which is important since humans don’t yet know what we want in the long run).
(You can find a list of all 2019 Review poll questions here.)
(You can find a list of all 2019 Review poll questions here.)