As far as I understand it, the agreement only covers that there will be a backstop, i.e., the exit agreement will have legal text that spells out a default option in absence of an agreement between the UK an the EU on how to handle the Irish border. However, the agreement does not cover what exactly that text will be. So, in case a miracle occurs and someone thinks of the magic solution that meets the apparently contradictory demands of the UK government, it could be inserted into the final text and replace the one that was proposed by the EU (if it is satisfactory to he EU as well). The way this is going, that miracle isn't going to happen, the final agreement will be close to the EU proposal and the current deferral is just another one of those that puts the UK into a worse position than it already is.
This is shaping up to be a very bad deal for the UK. It is going to give the EU almost everything it wants for a 21-month transition period. Once it is signed, the UK will still be under time pressure to negotiate deals for the future relationship, but the EU lost most of its pressure to sign a deal and can easily threaten to walk away from anything the UK proposes. The problem is that no matter how bad the deal, the UK will have to sign it or face walking off the cliff. A matter made worse by that the longer they keep stalling waiting for - what exactly? - the less time they have to prepare for that walk.
Article 50 was designed to place the leaving country in a bad position, but the incompetence and indecisiveness of the UK government has dug them into a much deeper hole.
Never underestimate Boris; he'll "much rather tell you the price for a glass of champagne"