Nice example - you were right, my wording on that section was incorrect. sukilot quite nicely presents the error in my derivation - I guess I probably shouldn't try coming up with proofs on the spot - especially if they're not machine checked.
I think what I probably should have said instead was that the low false positive rate means that only a small proportion of the honest URLs will be sent up.
I think what I probably should have said instead was that the low false positive rate means that only a small proportion of the honest URLs will be sent up.