Currently there isn’t a way to enable a syncing on a product review.
Generally in order for a customer to be synced to Mailchimp as subscribed there would need to be explicit consent from them to join the Audience.
While a product review is a good indicator of engagement with your product or content it is not exactly consent.
If you would like to provide more detail on what you would like to accomplish by being able to market to those that leave product reviews perhaps we can provide an alternative.
In the meantime look into Automations. This feature allows for those that do make a purchase to be sent some follow up emails automatically that remind them of reviewing and perhaps you can trigger other emails to send based on their engagment.
