Date-fns typings don't work?

I'm trying this code:

import { startOfDay } from "https://esm.sh/date-fns@2.29.3";

// No error?
let foo = startOfDay("foo");

import { startOfDay } from "https://esm.sh/date-fns@2.29.3";

// No error?
let foo = startOfDay("foo");

import { startOfDay } from "https://esm.sh/date-fns@2.29.3";

// No error?
let foo = startOfDay("foo");

import { startOfDay } from "https://esm.sh/date-fns@2.29.3";

// No error?
let foo = startOfDay("foo");

I can see that they use

declare module

declare module

declare module

declare module in their typings file, is this causing the issue?