The overall rating reflects the current state of the app. It accounts for all app reviews but prioritizes the most recent ones.
This app is only halfway functional and should be free until they fix the problems. If you use the "buy now" button on your store, the discount will not apply. It only works when the customer uses the "add to cart" button on the product page. It causes me to issue several refunds to customers who expected the discount to apply automatically.
I emailed their support twice but continue to receive nearly the exact same reply: "We recently acquired this app and have much to fix. Unfortunately that does not work at this time."
AFAIK they acquired the app in September 2018. That's almost one year ago. For a premium app ($4.95/month) this should be updated regularly.