You misunderstood. “Official” can be interpreted in different ways. There’s “official” in the sense that the Caddy team maintains the https://apt.fury.io/caddy/ repo, but it’s “unofficial” in the sense that it doesn’t ship alongside debian.
You probably installed it correctly, but you have two caddy things in your PATH. One from the package we provide, and one from some Python thing. If you do which caddy, you should see /usr/bin/caddy If you see something else, then it’s because something else is taking precedence over caddy the web server. If you run caddy version you should see something like this:
$ caddy version
It’s up to you to figure out what installed this other “caddy” thing. I’ve never heard of it.
I think I have found the other thing and uninstalled it
python3 -m pip uninstall caddy