I struggled with that too. Here's my mental model, which may or may not be relevant.
Imagine you're holding a penny directly in front of your eyes, between you and the sun. It can do a pretty good job of blocking the sun for you, because it's so close, but no one else is impacted.
Now move that penny a few hundred feet up. Now it's almost imperceptible, but far more people (technology permitting) are capable of spotting it.
Let’s say that you need to be within, say, 1 degree of the ecliptic. The height of that ring is much greater at 100 ly than at 1 ly, meaning more space for stars.
I was translating 'direct line of sight' as 'parallel enough to our ecliptic plane to see the Earth occluding the Sun'.
> Every star within 1000 light years is in a direct line of sight to every other star.
There could be space crap in between. - Of course I just realized something obvious. Along our PotE lies most of our solar system debris. Shouldn't that be obstructing alien views?
Excepting highly tilted orbits or unusually clean solar systems, how are we seeing exoplanets at all? Is there a sweet spot, just a bit above the plane, where observable solar occlusion still occurs?
> There could be space crap in between. - Of course I just realized something obvious. Along our PotE lies most of our solar system debris. Shouldn't that be obstructing alien views?
Space crap is a lot less dense than you might think. Generally speaking, flying through the asteroid belt edge on looks like... well, just flying through space.
Turn it around: the space crap doesn't really prevent us from getting good views of Jupiter, Saturn, etc. Right? And it wouldn't prevent us from seeing Mars or Venus if they were outside of the belt either.
The total mass of the asteroid belt is estimated at something like 5% of the Moon's, and a third of it is accounted for by Ceres alone. It's spread pretty darn thin.
The Kuiper belt is much more massive, but is spread out even more thinly. Same again for the Oort cloud, where the average spacing between comet-sized bodies is about the same as the distance from Earth to Saturn.
A favorite quote from a relevant reference work: “Space is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space.”
I’m not an expert but I don’t think this is true. The dark streak in the Milky Way (the “Great Rift”) is actually dust clouds that obscure our view of stars.
Think of the zone they're talking about as a disk: the plane of Earth's orbit, extending out to the maximum distance where the transit is still considered detectable. Most of that disk's area is far off, towards the edge; little of it is near.
If you're at a horse race, is it easier to discern the horses are running in circles from the sidelines, or from the bleachers? How about an overhead drone?
Imagine you're holding a penny directly in front of your eyes, between you and the sun. It can do a pretty good job of blocking the sun for you, because it's so close, but no one else is impacted.
Now move that penny a few hundred feet up. Now it's almost imperceptible, but far more people (technology permitting) are capable of spotting it.