A confidentiality agreement or NDA is fairly standard stuff in many sectors. It's just a contract and much like all contracts they can say anything they like, but whether they would stand up in court is another thing.
Courts like to use the word "reasonable" quite a bit. Therefore if your employer decided to sue you they would first need to prove that their contract was legally acceptable and then that this clause about unlimited liability was "reasonable".
It may be appropriate to ask a solicitor to scan the document for you. It shouldn't take them more than a chargeable hour, and may be money well spent. Maybe you could request an amendment to the clause to narrow it to specify under what circumstances they would have a claim against you if it should be leaked and the leak is shown to come from your office.
