The quote was pulled from a Tutorial and is not a standing rule.
Bounty Toggle Icons are not a free pass to join whatever thread you'd like, it's usually best to consult with the thread owner prior to trampling over a story. This is a cooperative role-play environment and the Bounty Toggle Icon is not some rule override to just start ruining stories - instead, it's an incentive and a "spotlight" for those looking for new role-play opportunities.
The only question that matters here, are Invasion threads considered public? Well, they're not private threads. But they're not marked public either. Invasion publicity parameters should be what the opposing owners of the Major Factions involved agreed on when settling their terms.
The quote has been removed and the tutorial clarified.
Judah Lesan
Ellie Mors
Scherezade deWinter