They use the word chamber, because it refers to the block, not the subsequent attack. The complete term would be chamberblocking, but most people can't be arsed to type it out completely.
You block the attack by chambering your weapon. Hence the name.
The attack that follows is just a normal attack as far as the game is concerned.
The fencing term would be a parry, followed by a riposte (stating the obvious, I know).
In short:
A chamber would be the block by chambering your weapon in a certain direction.
A ripost would be the attack that follows.