There's still many other images that the flare isn't affecting that area at all though, right? The point is that there's many different images used here, and instead of combining them in a way to remove transient artifacts, which is the norm, they combined them in a way to emphasize transient artifacts, which is not something people normally would want to do.

It's worse than any single image that's used to make it, so why make it at all?

It's an interesting way to demonstrate the density of the satellites- they're not hiding the number or duration of images, so you can get a fuzzy intuition for how likely you are to catch them in a given image.

Except you're not actually demonstrating the density. That could have been one satellite if you exposed long enough.

It's like taking a picture of the one car every day driving on some empty road and then coadding them and claiming you're demonstrating density of traffic.

what if the research requires long/multiple exposures.

If I understand correctly the satellites would be much dimmer there, but otherwise you'd see the same problem - correct me if I'm wrong.