You would be surprised how effective the paradolia effect is. Its very likely that if you had never been told what the words were, then playing it backwards you would have not heard any message at all. However, once you know what the message is, its clear as a bell in your mind.
This is similar to the case of the Fisher Price "Mommy cuddle and coo" doll which was supposed to speak Islamic messages. If you are not told what the message is, all you hear is cooing. After someone says "The doll is saying 'Islam is the light'" It seems to be clear as a bell.
The problem with the backward message is that it would be complete non-sense when played forward. It would not be unheard of for a band to pick up on an urban legend and plant a message like that as a joke, but I think the album was recorded before the "backward" message craze became popular. For example, Pink Floyd did some backward messages deliberately, however it was just jibberish non-sense forward.
The incredible amount of planning and coordination it would take to play a song that would sound good forward and have a clear message when played backwards is insane.
The message forward is an artistic song. Backwards is just paradolia at work.
The other problem with the "Satanic" messages in rock albums, is that it assumes the mind can somehow subconsciously play the song backwards and be influenced by the message. Research suggests that this is not the case. The mind has no such abilities, conscious or subconscious, and even if it could, research suggests direct marketing is much more influential than subliminal marketing.