Let's say the upstream server is apple.com. The TLS handshake is always performed by the real apple.com servers, and the ShadowTLS server is only a middle man forwarding raw TCP contents.

If both sides are ShadowTLS (client & server) holding the same key, they will stealthily switch to a different encryption protocol after the handshake, disregarding the TLS key exchange. The TLS handshake is a facade to fool the deep packet inspection of the censor.

In all other cases, such as the censor actively probing the ShadowTLS server, the server will keep forwarding the encrypted traffic to apple.com without anyway to decrypt it (it's not a MitM proxy). To the active prober, it is just apple.com all the way.