Would be a pretty big slap in the face for all other "contestants" if it turns out that Amazon intended to move to Atlanta all along and this whole thing was only for publicity and leverage.
It probably would cost amazon less to buy 10 potential properties and resell 9 of them, than to announce their intentions and then attempt to buy their chosen property.
In Atlanta, we already learned that lesson by announcing the BeltLine (mixed use alternative transit development corridor around the city) before acquiring any of the land. A decade later, after court battles involving the city purchasing land via the Board of Education in order to prevent further price gouging, we still don't have all the land needed, and only ~10% of the project is completed.