Your link makes it seem simple to handle. Ignore whatever PRIVMSG or NOTICE you send and then handle the reply from the Twitch IRC server as if it was your original IRC message.
Clients that receive self-sent PRIVMSG and NOTICE messages, MUST treat them the same way as if the client itself would have sent the message to the target. Clients may choose to disable local echoing of sent PRIVMSG and NOTICE messages altogether, or present them in pending state.
I however understand your point that is not standard regarding the IRC v3 specs.