Why does TypeScript not throw a compile error for an undefined type

Here's a simple repro to describe the problem:

1. I created a vanilla RN project using npx react-native init MyApp --template react-native-template-typescript
2. In App.tsx, I added let foo: IThoughtTheWholePointOfATypeSafeLanguageWasNotToAllowShitLikeThis = "...seriously, wtf, why does this compile without an error!?";
3. I run the app using yarn iOS

The app runs fine and there are no errors in the bundler.

Question: How do I make it throw a compile error?