OK, seems like they changed their policy at some point and it no longer mentions pro-rated refunds:
"Paid members who haven't used their benefits are eligible for a full refund of the current membership period. We'll process the refund in three to five business days."
"Paid members who haven't used their benefits are eligible for a full refund of the current membership period. We'll process the refund in three to five business days."
https://www.amazon.com/gp/help/customer/display.html?nodeId=...
I have received a pro-rated refund when I cancelled in the past.