Look at the amount of pages. Look at the private keys and how the vary very little in between each other, with only the last few characters changing. Now think about whatever your private key is, and how hard it would be to find your key specifically on a website that doesn't even let you search for the private keys and would force you to scroll through every page. The chances of you ever having your private key stolen are next to nothing (and they even explain that math makes it nearly impossible), and this is no different than doing a guess-and-check for private keys. Someone could try to do the same thing by themselves, it would never, or it at least is highly unlikely, to yield results.