But how do I take them with the simulator? The screenshots I capture are 2732 × 2048, which is not only too small but a different aspect ratio as well. Any ideas?

I found the solution in the following post: https://developer.apple.com/forums/thread/744385 To do that, go to your build settings and under Architectures, set Base SDK to 'visionOS'. Then you will see the Destination 'Any visionOS Device (arm64)' in Xcode. After changing the build setting, the binary shows up in App Store Connect as expected.