I can’t speak for the thought process of the copy writer. But anal is frequently talked about as another way of stimulating a woman’s g spot.

Its another way of saying YMMV and that there are more guys who DON'T get Greek with  her then do.  Hence, its a "secret" shared by a few.  In other words, she will do it if she's not intimidated by how much sausage you're packing, but she has to see you first, so there is no way to book it in advance and be guaranteed to get it.  

 
A Kgirl that will do Greek with anyone, even the magnums, is Yoona in LA.  The enema bag hangs right in the shower, so short-notice is fine with her, however the prep time for the cleanup is on the clock.