Had they said "the array will be so large it'll have its own gravity." then you'd be making a valid point.

But they didn't say just "gravity", they said "gravity well".

> "First, let us simply define what a gravity well is. A gravity well is a term used metaphorically to describe the gravitational pull that a large body exerts in space."

- https://medium.com/intuition/what-are-gravity-wells-3c1fb6d6...

So they weren't suggesting that it will be big enough to get past some boundary below which things don't have gravity, just that smaller things don't have enough gravity to matter.