Wow. Great question. I never noticed it before and it's right there in the ActionForm's javadoc.
Yes, if ActionForm is Serializable its subclasses implement Serializable.
Speople specify "implements Serializable" in bean subclasses as a gentle reminder. It's still weird that the Javadoc says that. Maybe it was intended as a reminder any non-Serializable class variable should be declared transient.