Duty-free purchased outside China, but bring into China with transit flight~

Greetings~

I heard there is not allow to bring any duty-free items (except for 100ml items) into China with any transit flight in between the departure country & China... Just wondering is it because we may have to go through baggage check (let's say transit in Hong Kong) that's why we are not allow to do so, or we only can purchase in their country itself?

I'm not sure how it works as I believe it should be all-right as long as I didn't check-out/ clear at immigration and with the plastic bag being sealed.....

Kindly assist me with my situation as my parents and their friends are traveling into China from Malaysia with transit in the next few months....

Thanks~~!!!