I will say that it was weird because at the time, I also tested loading the main page and a couple other blog posts on the site. Only this particular one, however, was an issue, and I figured it was probably an anti-hug mechanism where the site forces Brotli compression for a resource if that resource was frequently being accessed over a certain threshold.
I honestly have no idea though. It's weird. What I do know is that it wasn't just my browser - I was having a similar issue with wget, where it got back encoded Brotli data without having asked for it. (wget does support Brotli, but doesn't send an Accept-Encoding header or, it seems, decode the output unless told to specifically.) But, again, I'm not having the issue now, and as you say, not sending an Accept-Encoding header means anything goes so it's not really the same thing as how it worked on my browser.
I just re-checked, though, and it loads fine in my browser now; I'm getting "Content-Encoding: gzip" back. Perhaps it was a server misconfiguration?