Environment: (Settings you have changed (e.g. Auto Download). “Unusual” devices you use (e.g. Bluetooth headphones). Did you select another media player?)
Sorry, I hit post by accident before I was done. Then I solved my own problem before this post was approved.
Quick summary: a seemingly valid feed was getting the “podcast sent a website, not a feed” error. The problem turned out to be Cloudflare injecting a Captcha when I was using a VPN. Disabling the VPN let AntennaPod load the feed.