Open in terminal works, but Open in File Manager does nothing.
The reason for it is, that some time ago KDE blocked the basic GUI apps from opening as root from security reasons. However, to counter that, they added save/edit possibilities to file editors like kate and kwrite, so you can still work on root file from non-root user and then save the changes after giving root password.
The issue with file manager itself is, that they had to create some additional mode when the app was not in root but the actions were by default treated as from root (confirmed with password). This isn't done yet and I don't know if they have any plans or idea how to do it, hence the root action became useless at the moment.
Luckily (or not - depended on how you look at it) you can still use non-essential KDE apps in root, so Krusader root mode is still available.