Thank you for those insights, @leonick.
Can I propose that for all feeds with a date_modified but no date_published, Newsblur should interpret date_modified as date_published?
The .xml URL looks like a promising workaround. I updated the feed’s URL and Newsblur removed the feed from “All Site Stories.” But that might be a temporary condition, until new articles become listed in the feed.
In any case, a generalized fix for any feed having date_modified but no date_published seems appropriate.