People are forgetting All Along the Watchtower. Anders played it on Earth... it came from Earth, it's based on our reality. This is our planet. If they were all Cylons, then so are we.
Or - its set far in our future
or considering all our music and culture is being broadcast outward...
they could have come across it in some form and intergrated it.
as for everyone is cylon evidently there is some way to tell the difference since they determined the bones on the planet were cylon and not human.