I just got one of those remind to review mails. When you click on the link, the page lists a whole bunch of books you've recently purchased and not reviewed yet. A whole list of them. You can't just give stars, you have to fill in the title and review fields which show up as large blocks. And the "submit" button is grayed out unless you fill in those "blocks". There's a little note in there saying that writing something is "required".
Maybe this person thinks you have to fill out all the "blocks" in the list, for any of your reviews to be submitted? He probably has not gotten round to reading that one yet, and was reviewing one of the others?