baseURL?
I think that was it. The site was deployed with GitHub Pages, and “Enforce HTTPS” wasn’t enabled, so I think base_url in hugo --baseURL ${{ steps.pages.outputs.base_url }} was “http”, not “https”. I now see <atom:link href="https://willfaught.com/paige/atom.xml" rel="self" type="application/rss+xml"/> (note the “https”) live, so the scheme is now fixed! 
Thanks.
Edit: I put my other problem into its own post: RSS permalink is Atom permalink somehow