Remember the good cop, bad cop technique? Well, Google has always been the first one to show the maximum loyalty to developers. Unlike Apple, Google doesn't have strict requirements for design. The approvement period in the Play Market is much shorter and not as fastidious as in the Apple App Store too.
Nevertheless, in a question of smart assistant integration, it appears quite conservative. For now, Google assistant works with the selected apps only. The list includes such hot names as eBay, Lyft, Airbnb, and others. They are allowed to make their own Now Cards via special API.( According the cleveroad blog - https://www.cleveroad.com/blog/how-to-create-virtual-assistant-apps-like-siri-and-google-assistant)