It means what it says - N900 is not fully supported by PC Suite. And probably never will be as the current move is to put everything from PC Suite to Ovi Suite (tho Ovi Suite doesn't support N900 at all).

Maps Loader doesn't work in PC Suite mode on any Nokia device - you need to connect your device in Mass Storage mode to be able to use it with Maps Loader, and it works on N900 just like on any other N-Series device.