I just had a look and see that the PR hasn’t been merged. I’ll get someone to take a quick look at my changes and get them merged into the core code. Based on your questions, I’m not sure that the explanation I added to the plugin of how to set the feed topic’s categories will be clear though. I’ll try explaining it again here.
The category that the RSS feed topics gets published to is based off the domain of the feed’s link attributes, it is not based off the domain of the feed. For example, if your feed is at https://example.com/feed, but the link attributes in your feed are for posts at https://www.example.com/, the domain that you need to add to your Admin / Customize / Embedding hosts section is www.example.com, not example.com. The example below would cause all posts from the feed to be published to the “fun” category:
To find the value of your feed’s link attributes, you will need to look at the markup of your feed. You can do that by loading the feed URL in your browser.
It allows you to publish posts from a specific path on your blog to a Discourse category. For example:
This would publish all posts in the www.example.com/fun path to my “fun” category and all posts from the www.example.com/support path to the “Customer support” category.