It used to be evil Freon that was the problem; now it is Chlorine.
The chlorine comes directly from the freon released in the past. The freon is destroyed upon exposure to ultraviolet light but releases the chlorine which hangs around and breaks down ozone. How long the chlorine hangs around is debatable. Some say years, but I think it precipitates out. But like I said, most freon is long gone, only its constituent molecules remain.