Point me to the rule that states hidden cards can only target the battlefield they are hidden at.

According to Rule 811.1.d.2, when a card is played from Hidden, any targets chosen by its play effect must be selected from the battlefield where it was hidden:

"If a hidden spell or a play effect of a hidden permanent chooses any targets, those targets must be chosen from among options at that battlefield, unless the ability explicitly restricts targeting in a way that makes this impossible."

This is further supported by the RiftJudge FAQ #5706, which clarifies: "A card played from hidden at one battlefield must target units and place tokens only at that same battlefield, regardless of where the showdown occurs."