Description
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.
Comments
Do you know if it reads back
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.
You can ask Siri to review the post or message
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:
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.