You're seeing that because it's *tls.Conn, not tls.Conn, that implements net.Conn. See
http://play.golang.org/p/lc7dXbVqXk for a 'revised' example.
The wording of 'It implements the net.Conn interface' might be a little imprecise, but similar language is used in, at least, the UnixConn doc ("UnixConn is an implementation of the Conn interface for connections to Unix domain sockets.").