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. Your ads through a CSS partner 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 ads from dynamic remarketing, which is part of Smart Shopping Campaigns.
Such a clear impact of CSS, such as 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 benefit will generally not be fully and specifically represented in reducing the cost-per-click. But a thorough analysis will find it and sum it scattered across various indicators, such as increased volume and number of transactions.