I was wondering if you're using a phpBB extension you've built yourself, and if it's on GitHub or somewhere (the extension), or ... It's not a built-in feature?
Websearched for "phpBB federation" and "phpbb subscribe rss broadcasts", found this:
"Feed post bot: This extension enables you to read any RSS, ATOM or RDF feed. It looks for new items every half hour and post them to a specified forum." https://www.phpbb.com/community/viewtopic.php?f=456&t=241159...
Intering way to use RSS