source rss?

It’s not advertised at all on my site.

It’s usually still very simple to find the RSS feed for a site. Older versions of Firefox (for example) used to flag up RSS feeds on a site in its address bar. If your RSS feed url is being generated in the header of your site’s pages, then any feed scraper will pick it up immediately.

I thought RSS feeds or readers were not used by many people.

Some sites tend to publish feed from elsewhere. My best guess is that this is what has happened in your case. If you have access to traffic logs via your hosts, you might get some details on the the referrer (ie where these people are coming from).