This seems a bit exaggerated. You definitely don't need to start with certbot or even with a domain name. Why can't you just start with a regular server on localhost, which can be set up in way less than ten minutes, and publish it only when you're ready? Not to mention that Firebase Functions is way less beginner-friendly than Express.
Mobile app development requires an HTTPS connection.
Every React Native tutorial out there has a section on setting up user auth with Firebase, and then putting a few REST endpoints up.
It is simple enough that a beginner "never touched mobile or web" development tutorial can go through it in under an hour.
Firebase is incredibly simple to get started with.
Another solution is to use one of the HTTPS port forwarding services that takes a localhost server and gives it a public HTTPS endpoint, but that is more work to explain than
firebase deploy
so the tutorial authors go with Firebase. Auth being super simple is icing on top.
