The phrase "__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED" is a rhetorical and humorous warning from the React development team.
It's not meant to be taken literally as someone being physically fired from a job. Instead, it's an extremely strong and dramatic way for the React core developers to say that it's:
Extreme Instability
No Guarantees that it will work as expected, or even exist, from one version to the next.
Risk of Breakage
It's a clear message to discourage anyone outside the core React team from relying on it, because doing so will lead to an unstable and unmaintainable codebase.