I strongly suspect they didn't add a message on purpose:
1. They say so clearly in your quote. Why would they lie? There are many bands who play Satanic rock with Satanic lyrics, all in the open. _There is no reason for Led Zeppelin to lie!_
2. Pareidolia need to be accounted for. A serious study of back masking and pareidolia was performed in the early 80's and thoroughly debunked it[[1]](
3. Even the proposed lyrics don't really mean much: "whose power is Satan"? "He will give those with him 666"? I would expect a Satanic message to at least mean something and to be grammatically correct...
4. When Led Zeppelin wanted to speak about a controversial subject, they did so by using metaphor and double meaning, not by using back masking.