(IANAL and I haven't even read the document) My interpretation of that has been to safeguard against a smaller country or two dropping out: For example, if NZ

(IANAL and I haven't even read the document) My interpretation of that has been to safeguard against a smaller country or two dropping out: For example, if NZ bailed at the last minute, the agreement would still hold between the other 11 countries to whatever extent is applicable. However, if some critical mass of countries all drop out, the whole agreement is invalidated. In this sense, the negotiators didn't bind their countries to anything; they just set some terms regarding what does and does not invalidate the entire agreement.