From what I can see, this site sends your whole number to the backend to search for a number in the dump[0], while haveibeenpwned.com will hash the input, send only a prefix to the server and receive a list of hashes with the same prefix. If your hash is in the list, you've been pwned, but you can check without leaking your data to HIBP.

Edit: I just checked, seems like the form on the frontpage of HIBP also submits your complete email/phone number. Pretty sure I read about how you don't have to submit your personal data to validate against HIBP, not to long ago...

[0]: https://github.com/Fumaz/haveibeenfacebooked-api/blob/master...