They have an idea of how many people were required to do the work, and they found evidence of roughly that many people housed nearby who were fed and respected much better than we would expect slaves to be. Also, they found them exactly where they expected to find the slaves.

It's not the strongest evidence, but it seems like enough to displace the presumption that most of the laborers were slaves, unless there's equally strong evidence for that. I would expect the article to have mentioned such evidence, though.