That makes sense because Google says "de is for masculine and feminine nouns, meanwhile het is for the neuter" yet it's de appel oh boy!
Well, there's some truth to that, because Dutch did use to have three genders, masculine, feminine and neuter, but then the masculine and the feminine merged, giving us common (de) and neuter (het).
You can see Scandinavian languages like Swedish already went through this too (en/ett), while German still has three (der/die/das) and English obviously has lost all gender (the).
Appeltje is a "small apple". When you have "appeltje" you have to use "het" and when you have "appel" you have to use "de". Just remember that when you make a word smaller (word+tje) it is always "het".