Correct, The WMI calls is not available in the UWP app. When you convert your Win32 application to UWP app, you have permissions to access the WMI calls via the authorization.
A key goal of the Desktop to UWP Bridge is to separate application state from system state as much as possible while maintaining compatibility with other apps. The bridge accomplishes this by placing the application inside a Universal Windows Platform (UWP) package, and then detecting and redirecting some changes it makes to the file system and registry at runtime.
Converted app packages are desktop-only, full-trust applications and are not virtualized or sandboxed. This allows them to interact with other apps the same way classic desktop applications do.
So you can understand that the application packages it as an APPX and adds a UWP component. The app is still a win32 app.