I strongly suspect they didn't:

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.

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.