That's not what they're saying. The "chrome custom tabs" feature is a service interface exposed by an application, not the OS. There is no "open a custom tab" feature provided by Android. Therefore applications have to locate an installed browser that is known to support CCT before asking that application to start a Custom Tabs activity. The involvement of the OS in this interaction is to show the calling activity a list of applications that handle the ACTION_VIEW intent for URLs, and to bind the browser's CCT service to the interface for the client. The clients are the one choosing to only interact with Chrome at this point!