It's not a cryptographic key or a secret of any kind. In the case of React (or other JS libraries) export identifiers are chosen to expose private APIs for integration purposes with things like developer tooling and other functionality that isn't part of the publicly documented API.
Instead of choosing something like `__PRIVATE` the React library in particular chose something more eye catching: `__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED`, which is definitely sure to get people's attention when they look into it, but is essentially just a joke.
Instead of choosing something like `__PRIVATE` the React library in particular chose something more eye catching: `__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED`, which is definitely sure to get people's attention when they look into it, but is essentially just a joke.