Pharmacological doubts aside, there are other reasons to doubt the claim that people had for decades been turned into zombie slave labor. For one thing, the very process that would turn people into zombies (assuming it didn't kill them) would leave them brain-damaged, uncoordinated, and slow - in other words, hardly ideal farm workers.
Furthermore, the economics of zombie-making don't make sense: Haiti is the poorest country in the Western Hemisphere, with no shortage of very cheap labor to work farms and plantations. In a country where the average annual income is less than $2,000 there are plenty of able-bodied, non-zombified people willing to work for almost nothing. Unpaid zombie workers would still need to be clothed, housed, and fed, negating most of the potential profit from using them. And, of course, the sugar plantations allegedly filled with fields of zombies have never been found.
With the main reason for creating zombies pretty well debunked, the question remains - even if Davis's zombie powder is all he claims it is - why anyone would bother to make a zombie in the first place. It would be a lot of time and effort to abduct someone, fake their death, get the toxins just right, revive them, and put them to work.