Here’s how I understand it after a quick read through
L402 is not ActivityPub. It’s aimed at RSS feeds, and making podcast player apps that can access paid content.
So your server mixes paid/free content into a single RSS feed, then returns an HTTP 402 on unauthenticated requests for paid content. A smart podcast app will use this to let you buy the episode.
The FEP does a similar thing for ActivityPub requests. I think the two could work hand-in-hand.