What he means is that AUC cars can still come with faults, like wrong servicing and even non approved parts (brakes / wheels etc). So when something goes really wrong and a regional rep comes around, they might still disallow a repair even though a dealer sold it with AUC.
Unfortunately, you might believe the sales people you may deal with but that does not mean they are right and it does not mean that you are covered by warranty as it is often not the dealer, but BMW GB that needs to provide approval for repairs.