Jiang Chen didn't spend too much on the negotiation of "UA membership" since the two already agreed on the island. They were now just putting what they discussed onto paper.
As a businessman, Trump respected the contract; he did not renege after he became president. After he confirmed that the treaty was the same as they had originally agreed, he quickly signed his name on the document.
Based on the agreement, the UA would fulfill the commitment within two months, without any conditions, and submit an application to join the Earth Defense Alliance and leave the North Atlantic Treaty.
In fact, NATO only existed nominally the moment UA was defeated. An alliance that relied on Britain and France (Germany had already withdrawn) could not compete with the Earth Defense Alliance, especially after Celestial Trade mastered nuclear defense methods, the strategic balance between the two parties had long been out of balance.