WCG doesn't want people using an app_info.xml file. In fact, due to a bug in recent versions, BOINC ignores that the platform is not allowed and a fix is in place which will keep future versions from working with it if the project doesn't allow it. The app_config.xml is allowed though, regardless of the platforms allowed by the server, so that's the best option.

Then again, you may be able to include the platform element in the app_info.xml file to get it to work anyway. If that works, don't tell DA!