Why should we `synchronized(original)` since `original` is already thread safe?

ThreadSafeIntList.java

package je3.thread;

/**
 * Created by IDEA on 01/02/15.
 */
public class ThreadSafeIntList {
    protected int[] data;
    protected int size = 0;

    private static final int default_capacity = 8;
    public ThreadSafeIntList() {
        data = new int[default_capacity];
    }
    
    public ThreadSafeIntList(ThreadSafeIntList original) {
        synchronized (original) {
            this.data = (int[]) original.data.clone();
            this.size = original.size;
        }
    }
}