The written offer is part of the licence, as is the need to respond to that offer with the source code offered. It is all part of the same agreement.

A written offer on its own would not normally be directly enforceable in many (most?) jurisdictions, for the same sort of reason that retailers can't be held to incorrectly published prices (in the UK at least, a displayed price is an “invitation to tender”, not a contract or other promise) except where other laws/regulations (anti bait&switch rules for instance), or the desire to avoid fighting in the court of public opinion, come into effect.

But in this instance, the written offer and the response to that offer are part of the wider licence that has been agreed to.