It has always been the law that if you are on a visa allowing residence then you are required to exchange your licence before it is legal for you to drive. Only if you are on a tourist visa are you allowed to drive without exchanging your license.
Of course every other country allows residents a grace period because that is logical, but in HK it is a penalty to being resident.
If you got away with it, it is because they didn't understand the law they are supposed to enforce. Hardly something new in HK.