Now I am not an Oxford Latin tutor, but from knowing how Oxford tutors generally tick, here is my guess:
If you have not studied Latin but want to do classics, they are willing to take a chance on you and extrapolate how good you might be at it from their own aptitude test, and, probably, would like to see a Romance language score (A level from UK students, AP test from US students).
If you HAVE studied Latin, they will not be prepared to guess. They want to see what you can do with it. They want to see a test score. It doesn’t matter to them that the school doesn’t offer the AP class, because you CAN take the test without one, by self studying, taking an online class or dual enrolment. In fact, nothing will prepare you better for Oxford than self studying for an AP test because that is precisely the kind of independent worker they like to see.