In TypeScript, type guards are considered to be exhaustive, so if you have a string | number and check it against a function that says it returns "x is number", then TypeScript will think that in the negative case, it's a string.

If isInteger was marked as a type guard, then you could write code like this:

  function f(s: string | number) {
    if (!Number.isInteger(s)) {
      console.log(s.substring(0, 0));
    }
  }

which is clearly wrong.

There's an open feature request for a new kind of "one-sided" type guards that don't cause narrowing when they return false.