This does work for unencrypted DNS and DoT over port 853, but it fails for DoT over any other port and DoH, which looks just like a website visit. You can't tell whether a website is acting as a DoH server without TLS inspection, and you can't differentiate between multiple domains on the same IP address either (unless a client is stupid enough to use SNI). Some clients do actually hardcode direct IP addresses to get around entirely missing DNS servers, notably Windows.