This isn't due to tailscale not supporting or working with netcat; you can nc to an address on your tailnet just fine.

This is using the infrastructure tailscale set up for a different purpose. Rather than requiring a tailnet or even an account anywhere, you can tailcat between any two endpoint. Tailscale is there because it runs the DERP relays which get the client's pubkey to the server, brokers the connection, and coordinates holepunching.