You evidently cannot understand rules. That rule is about something completely different.
There is no rule against a new(/reply) author writing/editing whatever subject line they want for their own post. In fact, IV itself is the one that provides that capability with reply posts, and intentionally so, to make sure nobody is forced to decide between either 1) regurgitating somebody's incorrect or abusive subject, or 2) disjointing the discussion by not using REPLY to reference the post being referred to.
Writing/editing one's own subject line is not only NOT against the rules, it is a feature specifically offered to be used.