Still nothing. Proved that the groups.io RSS integration works with a reliable RSS feed, BBC News. Maybe a fresh post is required even for Comments on a post to be picked up so here goes. No change to these settings:
WP-Optimize exclusions to:
/feed/ — should cover Entries and Comments feeds
/forum/forum/* — should cover all publicly accessible forum feeds
AND
set Hostinger CDN cache to bypass the entire site