You can modify an offer, making a counter-offer where you strike out or rewrite the offending language. It's not a bad idea to initial and date the modification (Remember to keep a copy!). They can accept or reject the counter-offer, and in the latter case you would be out of a job. You could also put some written indication on the document that the attachment was not available to you, which would indicate (signatures and their statements notwithstanding) that there was not a clear agreement on certain issues (i.e. whatever will be in that document). Otherwise, the document states that you did read and understand, and you affirmed and accepted the terms and statements in the contract, so if you later breach one of those conditions (e.g. "not doing any freelance programming during the period of employment"), you could be in legal trouble for that breach. Their remedy for you not following a policy (which you couldn't know about) like "wear suit and tie", "eat lunch at the company store" would be at worst firing you, which they could probably do anyway. If there is a clause prohibiting freelance programming, you could be sued for damages (i.e. the revenue that they lost).
A theoretical greater risk in signing is that you would (apparently: don't know the contract language) be knowingly making a false statement, and it could be material to their offer that you actually read and understand the external document. Such a knowing material false statement qualifies as fraud, as "The suggestion, as a fact, of that which is not true, by one who does not believe it to be true". However, the jury instructions and civil code in California are phrased in terms of fraud in the offer, not in the acceptance.
That does not mean that an add-on disclaimer immunizes you against the consequences of a breach, it just provides a basis for excluding such terms from the scope of the contract. A conservative understanding of a contract is that it is what's in the four corners of the signed contract document, and if there are additional conditions that are to be binding on parties, such an addendum is supposed to be signed (as a way of establishing that the additional conditions are agreed to). An attorney with professional competence in California contract law could give you appropriate legal advice.