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.