Does voodoo have real power?
Yes. The context states that voodooism has intrinsic power; it is satanic power, but it is a power. It is tied to demonic activity, which is described as real and rampant in certain areas of the world, though held at bay where Christianity is strong.
Les explains that pagan religion steeped people in superstition and fear of the powers of their gods and the demons in the air around them. He says demonism is real; it was real at Christ's time and Paul's time, and it is rampant in other areas of the world. In Haiti, he notes, the people know voodooism has intrinsic power—it is satanic power, but it is a power. He adds that in America Christianity has been strong enough to hold demon powers at bay, though that is changing. The Corinthians' concern about demons entering through meat shows how seriously they took that demonic power.