There are two kinds of lists on Podchaser: lists of podcasts, and lists of episodes. Only podcasts can be added to podcast lists, and only episodes can be added to episode lists.
When you attempt to add an episode to a list, you will see only episode lists as options. Likewise, when you attempt to add a podcast to a list, you'll see the options for podcast lists.
Adding an episode to a list of episodes
Adding a podcast to a list of podcasts