If you have your basic project template where there is a file called SplashScreen.png in the Assets folder - you don't need to change Package.appxmanifest. Just rename SplashScreen.png to SplashScreen.scale-100.png and add SplashScreen.scale-140.png as well as SplashScreen.scale-180.png (at 868x420 and 1116x540 resolutions respectively) to the Assets project folder and the system will pick them up depending on current scale when the app starts. It is easy to test in the simulator (Project/Properties/Debug/Simulator, then hit Change Resolution in the simulator and rerun the app to see different splash screen images). I think the Package.appxmanifest editor does not allow to pick a splash screen file that does not exist, so if you don't have SplashScreen.png, but SplashScreen.scale-100.png - you won't be able to select it, but you can edit the manifest file in a text editor.
I think it is what they meant when they said in the article when they said "The splash screen image can be given as either a direct path to an image file or as a resource".