Here’s an update on this topic, we recently enabled ActivityPub based comments to be displayed and interacted with on https://podcastindex.org:
It uses C2S to fetch comments (reusing here the https://www.npmjs.com/package/threadcap library) and the OStatus way mentioned in Represent endpoint for remote interaction to trigger remote interactions in the instance of the “interactor”.
To avoid CORS issues, the comments must be fetched through the server instead of directly from the front end.