Depends, take a look at the rest of your settings in the Archiving page.
You might have set “Never delete Unread Articles”.
Also, that purge job is on a cron. I’m not sure how often it’ll run. Could be once a week or even month. Thousands of rss articles and links are only a few megabytes big. But you can push the “purge now” button at the bottom of the archiving page to check your settings.
Cool, did you install freshrss as a docker container, or as a package? I’ve also had issues with it not setting up crons and running them properly before too. Running the docker container helps, usually.