Hey, just another student here, but i believe both can be used interchangeably. I also have a feeling one of those is more common in the north and one in the south. Not a hundred percent sure about that though.
In general the classifier 'trái' is used for fruits, the classifier 'quả' for round or roundish objects. In the case of a standard apple both are acceptable :)
:P Actually, empty apples are real. In English called: Ghost apples: What The Heck Are These 'Ghost Apples' Spotted In Michigan? https://www.forbes.com/sites/robinandrews/2019/02/08/what-the-heck-are-these-ghost-apples-spotted-in-michigan/
Why is it not: an apple is empty? Maybe in general an untrue statement, but it may be a correct translation.
I am Vietnamese native speaker and he is saying mồn trái táo instead of một trái táo. Man, I don´t know why
I have heard this speaker pronouncing "mot" as something like "motn" in many cases in this course. I am now inclining a guess that it is a phonetic binding to the next word. In this case "mot trai" becomes a bump in the road. If said as "motn trai" it flows... Humbly, I am from Sweden - as far away as you can be from a native speaker.