Metropolis UI is a hybrid system that uses IPC with a WinRT process. The WinRT process is used to display tiles and whatever else is needed over in WinRT land. This can only exist on a system that runs WinRT. Which means Windows 8 and not Mac or the mobile platforms.
It is true that you can build and run Metropolis UI apps on Windows 7 so clearly they can exist without the WinRT part. But the entire purpose on Metropolis UI is to be able to launch fake WinRT lookalike Delphi apps from the WinRT subsystem. So they don't really make sense anywhere else. I don't see any reason why Embarcadero would have invested effort making Metropolis UI apps work anywhere that does not have WinRT.
What you could do is use the style part of an FMX Metropolis UI project. That would give you the visual look that you are after. It would not be a Metropolis UI project, but would look as you wish. So you make a standard FMX desktop application targeting OSX. Then apply your chosen Metropolis style. This certainly works for desktop applications and I see no reason why it won't work for an Android tablet application.