I think Caddy is serving the precompressed file after all. I ran
curl -so /dev/null "https://volpeon.ink/_astro/about.ArvwJ6Ft.css.br" -w '%{size_download}'
curl -so /dev/null "https://volpeon.ink/_astro/about.ArvwJ6Ft.css" -H "Accept-Encoding: br" -w '%{size_download}' and both commands yielded the same size.