take a closer look at the "Hey". it appears only on the "apple" (התפוח). but not in the "sweet". in heb in order to definite a phrase we add "Hay" to the object and its adjective. if there is more that one adjective we also add "Ve" " before.
the apple is sweet = HaTapuach Matok || the sweet apple = HaTapuach HaMatok || the sweet red apple = HaTapuach HaMatok VeHaAdom
Actually, the 's is short for "is", so his sentence is most definitely correct. If you have possessive in English, it can't be followed by an adjective. You also wrote "sweetness", which is a noun. "The apple's sweetness" would mean what you suggested, but "the apple's sweet" means the same thing as "the apple is sweet".