The .NET 4.0 folder contains the .dll and the 4.6.1 folder contains the xml... instead of them both being present in both folders.
The DLL doesn't get linked to your project when you add it.
Linking the DLL manually doesn't help. You still can't add the control (if you do it by CLR namespace it will show up in the designer at least, but still no xaml intellisense, and it always crashes at runtime during initialize component).
The last project release was 9 years ago. At that point everything worked fine. If you know how to fix the issue for 2019, please raise a pull request.