The inference is more like "people place sufficient amounts of infrastructure under direct control of a sufficiently capable AI" -> "there is no way to ensure that humans will actually be able to shut down the AI". My claim is not that this inevitably means that the AI will resist shutdown, or that it will inevitably take harmful actions, just that there is a nonnegligible chance that it could. The downside is large enough that even a relatively small chance is something to be worried about.