Product-Recommendations | Product Recommendations solution | Recommender System library

The reason why it did not work was same that you concluded, that it was not a product page. However, given the product ID we can get the recommendations of any product. The Shopify Docs for Recommendations object state

The recommendations object returns products only if it's used in a theme section that's rendered through an HTTP request to ?section_id=&product_id=. section_id is the ID of the section where the recommendations object is being used, and product_id is the ID of the product you want to show recommended products for. To determine the base_url, use the routes.product_recommendations_url attribute. Using the routes object rather than a hardcoded URL ensures that the product recommendations load in the correct locale.

So, unlike Shopify product page, where product object is globally available, you have to pass product ID from Shopify cart items. To do so, add a new section named product-recommendations-cart and include it in your cart template.