There is no way that Biden, who has long been an integral part of the Washington, D.C., establishment, is going to buck the U.S. national-security establishment. Biden made the announcement because obviously the Pentagon and the CIA authorized him to make the announcement.
As Michael Glennon, a professor of law at Tufts University details in his excellent book National Security and Double Government, it's the Pentagon, the CIA, and the NSA who are in charge of the federal government and foreign policy. They permit the other three branches of the federal government to maintain the appearance of power, but the real power lies with the national-security establishment. They are the ones who are ultimately calling the shots on foreign policy.
After all, look at what Biden did at the same time that he announced the end of America's forever war in Afghanistan. He announced sanctions on Russia, which has been America's Cold War enemy since the end of World War II.