Future vs future proche

Question

I wrote j’irai but Kwizbot said the answer is je vais aller

Céline Pickard · Accepted Answer

Bonjour à tous,
In this sentence, you cannot have "Le Futur Simple" because the English sentence includes "I'm going to [location]" which is the English Present tense and refers to an action very close in the future. Also, the time marker "this morning = ce matin" gives another sense of close future. If you are using Le Futur Simple in French, you are referring to something further away.
Ce matin, je vais au travail. = This morning, I'm going to work.
Demain, j'irai au travail. = Tomorrow, I will go to work.
 
I hope this is helpful.
Bonne journée !

Gunilla Öberg · Answer

I think this is the sentence Dawn is referring to. I have the same question "Ce matin, je vais aller au supermarché pour acheter des huîtres fraîches". Why not "Ce matin, j'irai au supermarché pour acheter des huîtres fraîches"