How does iCloud Photo Library's "Optimize iPhone Storage" setting work in relation to other photo apps on iOS?
It depends on where the app is pulling images from. If you have selected "Optimize iPhone storage" then these apps will only have access to the lower-resolution thumbnails, as bmike pointed out (they use Camera Roll). 

Unless the app is specifically coded to pull photos from iCloud Photo Libary (which Apple currently supports), they will just have access to the low-resolution photos stored locally on the device.

All the apps you have mentioned access your Camera Roll, therefore the low-resolution images.