You can call launchSignInUX from an HTML Article. The sign in UX that displays will depend on your authentication setup. If you are using authentication via the Direct Entitlement APIs, then the native sign in dialog will display. If you are using Custom Authentication with either a SAML, OAuth2 or Generic Identity Provider, then a web view with your sign in UX will display. If you include the HTML article as the first item in your Top Level Collection, it will become the Home screen and be the first thing a user sees. They can, however, navigate elsewhere.
We do have a "Sign In on App Launch" feature on the roadmap which will enable customers to require a user to sign in on app launch and not be able to navigate elsewhere until they have successfully authenticated. We do not currently have a release target for this year.