I've never seen any documentation describing the deployment of OEG or OAG on Weblogic server.
From what i know, it comes with an inbuilt server that is basically nothing but the gateway itself. It doesn't need any external application server for deployment.
I was having the same same knowledge the one that you supplied. We have set-up the API gateway with bundled server.
But I was working with Oracle support on one of the other issue and Support engineer informed me when you deploy the services you can select the WebLogic server instance like Gateway server instance. This was confusing so I open the Oracle ticket and post the question in forum.
I know we can integrate the OAG with WebLogic but installing the Gateway product on WeblOgic JVM process was my doubt.