The color and brightness of the glowing indicate the temperature of the dust. The temperature of the dust is only (essentially) affected by the light from the parent star, so the temperature depends on the distance from the star. So by looking at each star, even though they can't actually resolve the disks they can from the *color* of the disk determine how the dust is distributed around the star.
Next thing they did is determined the age of the stars.
One thing I'm skeptical about is the method they used to determine the age of the stars. For the most part the stars and the disks cannot be resolved. They get information about the disk from the IR spectrum of the star/disk combination. They get the age of the stars (according to the actual paper) from evolutionary tracks on the HR diagram - which rely upon the spectrum of the star/disk combination. Isn't there some circular reasoning there? Or are the evolutionary tracks based upon the visual spectrum and we're able to assume the visual wavelengths are entirely uncontaminated by the disk?
However, let's move on. Assuming they know the disk description, and assuming they know the age of the star, and they selected stars that are all like the Sun, they can then track how the disk changes with the age of the star. They clearly find that the disk thins out at an Earth-like orbital radius as the star ages. One possible explanation of this is that an Earth-like planet formed and cleared out the dust (by accreting it). It's also possible that the dust was just blown out by the stellar wind, but I am under the impression that the study ruled this out - that dust around and Earth orbit is preferentially dissipating as the star ages, not that all dust is dissipating which would be the result of a stellar wind.
So the conclusion: nearly all Sun-like stars form Earth-like planets. Very interesting. We have yet to *see* these planets, but there's evidence they may exist.