My impression was that running 10 Electron apps would be like having 10 separate installations of Chromium running at the same time (with no shared memory). But running 10 WebView apps would be like having one browser with 10 tabs open, but where a lot of the the browser resources are shared.

In that case, I would expect a large difference in memory footprint, although both options would be bad in terms of having a non-native user interface and user experience.

Is this incorrect?

The native webview use another instance of the engine, without running a complete web browser. If all apps did, the amount of resources required would be lower as they would share some common components.