I strongly suspect they didn't add a message on purpose:
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!
Pareidolia need to be accounted for. A serious study of back masking and pareidolia was performed in the early 80's and ...
Researching this a few years back I came across a site which demonstrated that in most cases like this, you can only hear these lyrics if prompted with them in advance. Otherwise you just hear gibberish or some other 'my brain trying to make sense of this' words.