Unhackable server?

I recently found a server called Zombie Horde (pretty sure I'm not allowed to link it but a simple google search will show the website) that appears to be immune to hack clients.

I have tried over a dozen Minecraft hack clients now of many different 1.7.x versions (including the latest) and all of them simply refuse to connect with the error:

Interestingly enough, vanilla Minecraft 1.7.9 and 1.7.9 with X-ray are able to connect just fine.

My question is: is anyone else able to connect to this server with a hack client, or is this server truly immune to hack clients?

I've gotten that before, I never paid much attention to it. I will do a bit of research on it.