The internal laws regarding treaties are irrelevant as to what constitutes a treaty under international law and the 1969 convention was merely codifying long-held standards.You are probably looking at the post-1969 definition. Furthermore, treaties need to be ratified. In most countries, including the United States that was a party to the Instrument of Surrender, the legislature (or part of the legislature) is required to ratify the treaty. This did not happen with the U.S. nor with any of the other Allied signatories of the treaty. None of the Allied powers regard the Instrument of Surrender as being a treaty.
Obviously your refusal to accept a treaty as a treaty is just your little dance to satisfy your misguided position. Not only was China's sovereignty over Taiwan recognized by treaty, independence was not even allowed as an option like it was with Kosovo. That it was a treaty is obvious by the behavior of the parties. Not a one challenged Taiwan's status as Chinese territory. All have consistently recognized Taiwan since the surrender as part of China. No mention of transfers of sovereignty were included about any territories held by Japan in the peace treaty. Obviously all recognized that the legal transfer of sovereignty had been required with the surrender and took effect when the Chinese military took control of Taiwan. If there had been any doubt about this it would have been included in the peace treaty.According to the recent ICJ advisory opinion, that would seem to be the case. However, Kosovo was a part of Serbia as confirmed by by the Treaty of London. There is no such treaty that confirmed Taiwan as being part of China.
Their actions clearly speak to the fact that it was a treaty and Taiwan was thus ceded by Japan to China.