The issue here is that typeof value === T - a generic type.
A generic type is "not a nullish type" - it is just an amorphous blob thing.
Which our code doesn't know how to handle.

Another way to implement this is as follows, which no longer requires the non-null assertion.

function nonNullish<T>(value: T | null | undefined): T {
  if (value == null) {
    throw new TypeError();
  }
  return value;
}

I have a similar case which I think is in the same vein as this one.

function foo<T extends string | number>(val: T) {
  const calculated = typeof val === "string" ? 0: (val as number);
  return calculated !== undefined ? 0.6 * calculated : 0;
}

The val type is T, and it's not subject to control flow narrowing, so if I remove the as number, 0.6 * calculated will fail.