From the QA link: They can also be in other forms, such as a bar, but if they are, information on their label must not represent the product as a conventional f

From the QA link: They can also be in other forms, such as a bar, but if they are, information on their label must not represent the product as a conventional food or a sole item of a meal or diet. So, Soylent is probably considered a food since it is a sole item of a meal.