The author of this clearly mostly runs non-static sites in containers, but since they have container-based hosting setup already it's more consistent for them to deploy a static site with Caddy in a dedicated container than to spin up a new deployment mechanism that's inconsistent with everything else they are running.

Also their site isn't entirely static: they're using Caddy to proxy specific paths to plausible.io for analytics.

That’s what I’m saying. I can use nginx/caddy/whatever to proxy specific paths just the same, with a single server instance. Some of my static sites are even protected by SSO, with a shared auth endpoint proxied by nginx. Proxying wasn’t invented after containers after all, that’s basically an orthogonal concern here.