Hi Rex,
We call the same shell API to retrieve the FileSystemPath for those logical "library" folders but the calls return an invalid argument HRESULT for them, so they result in null value in our API. It seems that those libaries might not have a dedicated file system path like "Documents" (local documents) does.
If you go to the "This PC/Documents" (which is probably the documents library) and try to edit the path in Windows Explorer, you'll see there is no real path. I believe they mock up a fake logical path in that app.
Whereas if I enter "::{031E4825-7B94-4DC3-B131-E946B44C8DD5}\Documents" (the ParsingName) in Windows Explorer's breadcrumb path, it will go to the Documents library.