That is not how "embedded" works. An "embedded" container is not a repackaged version of the server distribution stuffed into the CAS web application artifact. You won't find a "real jetty" if you unpacked CAS. Embedded container only means that you are getting a server that is able to run the CAS webapp automatically without extra manual/download work, regardless of how and to what effect.
If you want to actually use a real jetty instance, download it first, remove the embedded jetty from the CAS webapp build and deploy the now-made-vanialla/plain webapp there as you normally would with any other webapp. With embedded, you lose control at the expense of auto-configuration, automation and comfort. If you prefer manual work for deployments, embedded is not the right option for you.