The message Flow-preview-warning currently says:
You are seeing a preview. Submit the form to finish posting this, or click "Keep editing" to continue writing.
This is a bit too technical. Flow is supposed to make things very user-friendly, so it shouldn't talk about "submitting a form", which sounds very HTTP/HTML-based.
It should probably mention the button name and say something like "Click "Submit" to post", or better yet - say nothing at all and make the interface clear without the need for an explanation.