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