What Albert is saying; the keys aren’t ‘stored’ in some list on that website or any other website like it.

When a user clicks on a page, the website uses your cpu power to generate, the whole Bitcoin priv key to public address, at that time, on the fly.

Think about it, there isn’t enough memory/hard drive in the world to hold 2^256 keys, pub address, etc.

Using that site is pure brute forcing, page by page, which is eleventy million billion times slower than current programs.