The United States Futsal Federation Laws of the Game book does not
mention if a goalkeeper can or cannot score a goal by throwing the
ball directly into the opponent's goal.

If the ball touches someone, of course, the goal should be validated.

My experience as a referee/player/coach is that a goal cannot be
allowed if a goalkeeper throws it directly into the opponent's goal.
This question actually came up during a referee's course and the
instructor's answer was that the goal should not be granted.