someOtherVariable doesn't trigger an error!?

UnicornEditor.js

type UnicornEditorState = {
  editorState: any,
}

export default class UnicornEditor extends Component {

  state: UnicornEditorState = {
    editorState: EditorState.createEmpty(compositeDecorator),
    someOtherVariable: 'Should throw an error?',
  };
  
  render() {
    return (
      <div>
        Hello World
      </div>
    );
  }
}

Do I understand correctly that when defining types it's not strict and all kinds of subtypes are still valid in Flow in general?

Mostly right, yea. It boils down to variance:

class Foo {
  someProp: {bar: string};
}
let foo = new Foo();

// This assignment is safe
foo.someProp = {
  bar: "string stuff",
  baz: 42,
  blah: new Date(),
};


// This assignment is safe
foo.someProp = {
  bar: "string stuff",
};

// This is safe
let str: string = foo.someProp.bar;

// This is not safe because the `someProp` property type includes *all* objects 
// with a "bar":string property (but not all objects with a "bar":string property
// have a "baz":number property)
let num: number = foo.someProp.baz;