Thanks to @yutakahirano I realized that https://mikewest.github.io/corpp/ doesn't describe how the cross-origin isolated bit gets set for agent clusters of shared/service workers.

For documents this happens because the browsing context group has that bit set (due to COOP, primarily) and then each agent cluster that belongs to that browsing context group also has it set. (https://gist.github.com/annevk/6f2dd8c79c77123f39797f6bdac43f3e#changes-to-navigation step 6.ii. at the moment.)