Islands is just a catchy name I guess. I always thought the markojs terms for it made sense, but are more technical / less catchy: they called it “full page hydration” -> everything needs to be delivered as js and “component level hydration” -> islands, only specific component sub-trees need to be hydrated.

Then “sub-component level hydration” would be resumability like in qwik where only events and their dependencies get serialised as client js.