1. Is there anything else that unlocks the native resolutions?
No for older projects you can only unlock native resolutions by adding launch images
2.I've made a new empty project that does not have any assets and it is working with the native screen sizes.
If you create project from XCode 6 naive resolution will be automatically enabled even if launch images are not added. I just created new sample project from Xcode 5.1.1 and than ran it on Xcode 6 and naive resolution was not enabled and views were scaled in order to fit the screen.
Hope that clears the confusion as your project are old as you mentioned naive resolution will not be enabled unless you add launch images.
Correction
Based on comments i am correcting my answer for projects created in XCode 6 you can delete
Launch Screen File
and also not add any iPhone 6/6+ specific launch images. The app will not run in native iPhone 6/6+ resolution.
So For iOS8 you can have either Launch images for iPhone 6 and 6 Plus or a Launch Screen File
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