On iOS, if VoiceOver is on and you request Siri send a message or tweet, it reads it back. "Here's your message to Alice. It says, hello there, Alice. Ready to send it?" This way, VO users know what Siri interpreted. On Sierra, though, Siri doesn't take VO into account, and simply asks if you're ready to send the message.
Bug First Encountered
How often the bug occurs
Interact with the 'conversation' scroll area in the Siri window and find the card with your post/message. Review it, then use the button in the card to send it.
Do you know if it reads back texts and tweets when Voiceover is turned off? If so, this would be a bug. Otherwise, this sounds like a feature/enhancement, as Apple may have intended for it to behave this way.
When Siri asks you whether you are ready to send a message or post, and you don't reply yes or no, but instead say review, not very intuitive though, you may be able to have Siri read it back depending on its type:
- If it's a post like tweet or Facebook status, Siri will reply, for example, "Your tweet says," and then read its text.
- If it's a message in the Messages app, Siri will reply Okay, or, in some cases, "Your message to <somebody> says," but will not actually read the message afterwards. Which is most probably really a bug.
- If it's an email message, Siri will not understand your request to review the message at all.
I think it's not a bug that Siri does not read the message or post back right after dictating it like on iOS, which is also how Siri behaves on both iOS and macOS if VoiceOver is turned off. It just shows that Apple's main focus concerning accessibility is on the iOS devices, what can also be seen in other apps,, such as Photos. However, yes, I would also prefer if she read the message immediately, with an option to interrupt her with the Siri shortcut (Command-Space bar), saying "send it" to quicly confirm the posting, just exactly like it works on iOS.