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](http://www.darklyrics.com/v/venom.html), 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<sup>[[1]](http://books.google.co.uk/books?id=b39Udl6qY30C&lpg=PA78&vq=backmasking&dq=isbn%3A0805805087&pg=PA78#v=onepage&q&f=false)</sup>.

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](http://www.azlyrics.com/lyrics/ledzeppelin/thelemonsong.html), not by using back masking.