I'm not even getting that from the link. To me, the link only shows that I can download The Haskell Platform for my Mac.
The only thing there related to XCode is that one needs to have it installed as a prerequisite for running the haskell platform's installer.
This is no surprise because the installer wants to have the "unix development support" package available (it contains gcc, header files, misc command line tools in locations where run-of-the-mill Makefiles can find them...) and it's just incidental that one needs to run the XCode installer in order to get these basic unix amenities on the system.