I think the key point is the event horizon, or effective boundary, is relative to the observer. From the earth there’s a point beyond which we can never observe

I think the key point is the event horizon, or effective boundary, is relative to the observer. From the earth there’s a point beyond which we can never observe, even if we left today at the speed of light. However for a planet at that point everything looks “the same” to them - except the earth is at their boundary. The boundaries here aren’t absolute - they’re only relative to the observer.