A Hermes manager told me a while ago that this is actually true. Apparently if you buy enough other stuff at the store they are more likely to produce a birkin from a magical back room. But who knows I haven't tried.
This whole thing cracks me up when I think about the Hermes location that just opened in Charlotte, North Carolina. I can't imagine the "secret stash" game going on in the backroom in lil 'ol Charlotte. Charlotte thinks it's a bigger, edgier, well-to-do city than it really is.