To modify the output of the
digest-list tag, you’ll have to use the
bbpnns_digest_format_forum_posts hook indeed. The link you mentioned has the format of the parameter passed. Among other things, you get the post_id and post_type (topic or reply) which you can use to fetch the post content and build the digest however you want.
To get the post content, you can use the
bbp_get_reply_content() functions provided by bbPress.
I hope this helps.