Lack of a signature doesn't mean anything: what matters is what the agreement is. If they've offered you a position subject to certain conditions and you have accepted, then you have a contract, which is enforceable. Accordingly, "except as otherwise provided herein" becomes rather important. For example, it would be important to determine if there is a clause that allows you to reveal that you are subject to a confidentiality clause: if not, your question constitutes a breach of contract. If someone asks you about your salary, you can't even reveal that you are obligated to not answer (unless it is permitted). However, this is probably not what the employers intended and instead they probably just want to prevent salary comparisons. To minor extent, this confidentiality clause is not legal, since you are required to report income to the federal government, but perhaps there is a clause that says "except as provided by law". The consequence of breach would no doubt be simply getting fired.
In light of the fact that there isn't yet a contract since you haven't accepted, you are not required to keep quiet about the salary, or anything else in the contract. It does raise an interesting question as to any obligation to reveal that you spilled the beans to someone before accepting (if you do accept). It would be reasonable to conclude that they absolutely do not want you talking about your employment relation so you should say that you discussed the contract with someone. But it would also be reasonable to conclude that their interest is in preventing employees from comparing salaries, and your purpose in talking about salary is something unrelated, so you would have no reason to think that the confidentiality clause is relevant to you until after you are an employee. This is a situation where you should actually hire an attorney to look at the contract and advise you based on the contract and state law.