This might also be an OSX issue as I have Win7 64 and my project files load the actual project when double-clicked.
I don’t know how to change settings in OSX but if your event handler is changed to load Sublime Text and pass the project as variable that may change the way it is opened.
Although, as I write this I am also looking at how the registry is setup to handle that type of file and the actual program (Sublime Text) is important.
ie. in the registry both Notepad and Sublime Text can be used to “Open with …” that type of file, however Notepad doesn’t know what to do with it and just loads the project file as a text file whereas Sublime loads the actual project.
So I fear it may be the OSX version of Sublime Text that doesn’t handle them the expected way.