I presume the details of this post got lost somewhere?
let x = () => ({ name: "Alice" });
let y = () => ({ name: "Alice", location: "Seattle" });
x = y; // OK
y = x; // Error, because x() lacks a location property`
https://www.typescriptlang.org/docs/handbook/type-compatibility.html
There doesn't seem to be an article directly on subtypes