The spec is genuinely open (ucp.dev) - anyone can implement it without being a partner. Just publish a JSON file at /.well-known/ucp.
Built a free validator at ucptools.dev to check implementations.
Tested several "partner" sites and almost no one has deployed it yet, including google.com (returns 404). The 60+ endorsements are just that - endorsements, not implementations.
Take a stroll through the article, you'll read that its an open standard meant for anyone- quite the opposite of your quip. This works for any retailer.