Any user can edit any list.
I hope you guys doing great out there.
While i was hunting Instacart ,
I found that any user from whom a list is shared via link or any random list is editable by just adding
edit parameter to the list for example:
https://www.instacart.com/store/whole-foods/lists/xm7tfw is my list which cannot be edited.
But if you add edit parameter like : https://www.instacart.com/store/whole-foods/lists/xm7tfw/edit
But it is limited to adding product to the list .
#Nouman_Malik Sorry You went duplicate.
All the best for future bug hunting.