One thought I had is that one could use this layout engine to do the calculations for the positioning of visual objects on the screen, but you would have to essentially build a fully integrated layout manager that renders to the particular framework you're using. Maybe overridding the hooks for existing layout managers that come with the particular UI framework (for example, WPF has layout managers).
For example, in WinForms, if you build the UI programmatically rather than old fashioned drag-and-drop, you could set X/Y positions, widths/sizes etc. per the constraints generated by this engine. Needs to be observable/reactive so the UI changes are fed back into the engine and then the UI automatically changes - much like existing layout manager tech.
Perhaps doing this work/exercise was left to the open source community/reader. There aren't any examples/tests showing how to integrate this layout manager with any existing .NET UI widget frameworks. It looks like they wrote the integration with .NET in C# (on MacOS as well), and tested the layout engine itself, but that's about it.
EDIT: Possible hooks for old WinForms maybe? https://msdn.microsoft.com/en-us/library/z9w7ek2f(v=vs.110)....