This example from the article looks like an argument for immutability to me.

    interface A {
        x: number;
    }
    
    let a: A = {x: 3}
    let b: {x: number | string} = a; 
    b.x = "unsound";
    let x: number = a.x; // unsound
    
    a.x.toFixed(0); // WTF is it?