> they would have to do their own client-side TLS handshake
By intercepting the STARTTLS port the attacker can merely decrypt -- rather than, as you wrote, decrypting and re-encrypting.
> Traffic dump on port 5222, the connection is hijacked on application level (L7), the server receives replaced ClientHello message from the client.
> they would have to do their own client-side TLS handshake
By intercepting the STARTTLS port the attacker can merely decrypt -- rather than, as you wrote, decrypting and re-encrypting.