Looking at the example here: http://docs.oracle.com/cd/E18941_01/tutorials/buildmobileappscontent/adfmobiletutorial_1.html
Would doing something like this end up creating a package app(like an APK on android), or a mobile application. If not, can I create a native(ish) app with Oracle ADF? And if so, can I easily create a mobile app from a pre-existing web ADF app?
No, you can't use a pre-existing web ADF app to create a mobile app. You can however reuse parts of it, for example by exposing your model. It also uses the same concepts (like task flows) so the learning curve is low if you know ADF.
The end-result of your ADF Mobile application will be an .apk (Android) or .ipa (iOS) so yes, you get a native application.
Note: ADF Mobile is 'deprecated' (= will not get any updates/fixes). The product has been rebranded to MAF (Mobile Application Framework) and has lots of improvments compared to the version you are using.