The article does not say that. They seem to have implied this by mainly the evidence the builders ate a lot of meat, but their own assessing that the lodging was absurdly dense says otherwise. Sounds quite a bit like cherry picking to me, which is not uncommon for people working on the same subject for decades. At some point you are just too willing to accept your own confirmation bias, no matter how trained you are.

And this is just me guessing from the article, there are other reasons to think twice before reaching such conclusions. The concept of "slave" is not as set in stone as we think, it's a generalization of a broader concept.

And cattle is not necessarily for the rich: livestock is a pretty good way to store energy. In hot places like the tropics, vegetables go ripe very quickly so the logistics of feeding an army of builders with crops might have been difficult enough that it was just easier to feed the crops to cattle then move the cattle to the people.