This article from Microsoft: Guide to Universal Windows Platform (UWP) apps states that you can convert your classic desktop application (like Win32, Windows Forms, and WPF) or game to a Universal Windows Platform (UWP) app or game
.
Can the same be achieved for a Silverlight
out-of-browser application?
Unfortunately there is no easy and fully automated way to port your Silverlight app to UWP mostly because the API footprint has changed and updated between the 2 platforms. There are a lot of similarities but also a lot of breaking changes that require some manual changes.
Not sure if this is the article you meant to reference in your question but here is Microsoft's guidance to this upgrade path.
From the article above:
Note This guide is designed to help you port your Windows Phone Silverlight app to Windows 10 manually. In addition to using the information in this guide to port your app, you can try the developer preview of Mobilize.NET's Silverlight Bridge to help automate the porting process. This tool analyzes your app's source code and converts references to Windows Phone Silverlight controls and APIs to their UWP counterparts. Because this tool is still in developer preview, it does not yet handle all conversion scenarios. However, most developers should be able to save some time and effort by starting with this tool. To try the developer preview, visit Mobilize.NET's website.