关于 TypeScript Truthiness narrowing 的一个疑问？

TypeScript 的 Truthiness narrowing 有如下介绍：

all coerce to false, and other values get coerced to true. You can always coerce values to booleans by running them through the Boolean function, or by using the shorter double-Boolean negation. (The latter has the advantage that TypeScript infers a narrow literal boolean type true, while inferring the first as type boolean.)

// both of these result in 'true'
Boolean("hello"); // type: boolean, value: true
!!"world"; // type: true,    value: true

当我键入以下代码时，类型提示却都是 boolean :

let x = Boolean('hello')
let y = !!'hello'

这是为何？