this secret hidden rule need to take place?

What secret? How is it hidden? It's posted publicly and announced before they even posted the full rules on battles. Are we not supposed to know about it? Who is hiding it from me? Are they going to be pissed off I figured out the secret? Should I not tell Wizards that I know? Am I not supposed to know this secret, hidden information?

"Creatures can attack" isn't written anywhere on the cards. Is that a secret? Do people not know about that?

If that doesn't need to be written for people to know how creatures work why does "Battles can't attack even if they're creatures" need to be written on the card for people to know how battles work?

"Spells can't target themselves," "Auras go to the graveyard if they aren't attached to anything." These rules are also designed to prevent unintuitive situations and are not written on any cards as far as I know. I wouldn't call them secrets though.