They use a two-level hierarchy of topic categories. Well, it's multi-level but on the front page it shows only two levels. The topic categories make it clear their index is broadly general covering all topics.
Presentation of a podcast lists all episodes on one page. This would make for an exceedingly long page if the episode descriptions were long, but the podcast pages I've looked at on podcast.net had only short episode descriptions. This may be a mistake on their part, because the search engines may have a hard time figuring out how to direct visitors to visit those pages.
Anybody can add their podcast to the directory using a link at the bottom of the front page.