When you score your 8th point outside your turn, do you still need to control all the battlefields?

Yes, with a key clarification: the rule is about scoring (not just controlling) all battlefields that turn.

Ruling: To win by scoring the 8th point on an opponent's turn, you must conquer every battlefield that same turn. A single conquer on the opponent's turn will only draw you a card instead.

Here's why: The only way to score outside your turn is by conquering — Hold only happens during your Beginning Phase (Rule 464.2). And Rule 466.1.b.2 says that when you try to gain the winning point through a Conquer, you must have Scored every Battlefield through either method this turn — otherwise you draw a card instead of getting the point.

Since you haven't held anything this turn (it's not your turn), you'd need to conquer both battlefields on that same opponent's turn to win. Conquering just one puts you at 7 while drawing a card, no win.