Your source doesn't agree with your statement.

from the wiki page: In addition to the many unresolved arguments about the construction techniques, there have been disagreements as to the kind of workforce used. The Greeks, many years after the event, believed that the pyramids must have been built by slave labor. Archaeologists now believe that the Great Pyramid of Giza (at least) was built by tens of thousands of skilled workers who camped near the pyramids and worked for a salary or as a form of tax payment (levy) until the construction was completed, pointing to workers' cemeteries discovered in 1990 by archaeologists Zahi Hawass and Mark Lehner. For the Middle Kingdom Pyramid of Amenemhat II, there is evidence from the annal stone of the king that foreigners from Palestine were used.[1]

Couple points. You can be both a skilled worker and a slave. Also, if you are working to pay off a tax debt, that pretty much makes you an indentured servant or slave.