I'm not sure this will work and haven't tried it, but offering as an idea/suggestion.
It's possible that Samsung's "Touchwiz Home" is what forces the default app association. If this is the case, you could disassociate the Podcast Player from Samsung's Media Player only by installing another Home Screen Replacement app such as Nova Launcher and then making it the default Home Screen:
https://play.google.com/store/apps/details?id=com.teslacoilsw.launcher&hl=en
Then go back into these settings and see if you can make the adjustment.
If you still can't make the adjustment when using a Home Screen Replacement app, try downloading default app manager app:
https://play.google.com/store/apps/details?id=com.appiator.defaultappmanager&hl=en
There is also a pro version:
https://play.google.com/store/apps/details?id=com.appiator.dampro&hl=en
The app description lists this feature:
* Wide range of categories consists of Audio, Audio Streaming, Browser, Email, Home Launcher,
Messaging, PDF files, Phone Dialer, Video and Video Streaming
But again, I haven't tried this for myself!