I don't think this is totally true.
There have been several instance of people mounting the satellite antenna pickup on the dash or on the rear deck such that it has window view to the sky. It's not as reliable as a roof mount, but it works.
In fact, I read about some dude whole actually mounted his XM antenna UNDER his rear deck so you couldn't see it at all. It still worked because he mounted it under the deck in a place where no metal was above the antenna (only plastic, foam, and glass). He said this worked just fine.