Thanks for reporting this.
I have made my own little investigation and I found a very nice feature in silent mod.
You can use LUA support and one of the commands is :

et.G_Damage( target, inflictor, attacker, damage, dflags, mod )

Does amount of damage on target inflicted by inflictor and cased by attacker.
I am quite sure that's the trick being used on that server.
And I would like to warn anyone else who is thinking of using this trick.
It is not tolerated and will be dealt with.

Once again I would like to express my appreciation to the people who are actively guarding the service we are offering !