We can detect them spread out over an area, as shown in the article, not just coming from a single point each. If they were point-like sources, we would see them as a dot in the radio frequency data, like a distant star visually.
Yes, but the thing is we are viewing it at a single location (Earth), and it appears to come from an area, not a single point. The only way to get what we're seeing from a point source is for it to spread out in straight lines in all directions from the source point (as radio waves do, just like light), then somehow change direction towards us to converge back together here, appearing to come from multiple directions. That doesn't happen. (Gravitational lensing can change light's direction in some circumstances, but astronomers know all about that, so they'd give us that explanation if it made sense for this.)
For example, imagine looking across a dark room at a single LED, a point source. You see a point. But the LED is sending light straight in all directions, not just at your eye. The light going in other directions doesn't reach us, so we don't see that "area".
Imagine it's a white computer screen, an area source not a single point, and you can see the rectangular area. Every point on the screen is sending light spreading out straight in all directions, but for every point on the screen one of those directions reaches our eye, so we see the area. The light is coming to our eye from multiple directions, from the different parts of the screen, making the area.
Now imagine reflecting objects at various locations in that room, and an obscuring object in the line of sight so you can't see the LED directly. You can see light from that single point source from various locations in the dark room, but cannot see the origin.
That's what the GP is asking, I think. How do we know the patterns we see aren't reflections or interferences that could be traced to a single point source?
I don't think there exist enormous radio-reflecting objects in space, let alone ones that let through visible light perfectly. And there are many of these circles, so it seems unlikely that all of them could have their source obscured.
