This discount is not an actual reimbursement of CPC costs, but an advantage in real-time bidding for a specific bidding position against Google CSS. If you work with a CSS partner, your ads will start from a better position, participate in more auctions, and are likely to get more impressions. CSS partners do not have a price advantage over each other.
They also do not have a price advantage in dynamic remarketing ads, which is a part of Smart Shopping Campaigns.
The obvious benefits of CSS including a cost-per-click reduction will only be seen in a standard CPC bid shopping campaign. In a smart shopping campaign or a simple but bid-based conversion strategy, the benefits will generally not be fully reflected by reduced cost-per-click. But a thorough analysis will find the benefits as scattered across various indicators, such as increased volume and number of transactions.