You can, though it's usually useless; but it also depends on whether that subcategory is always necessarily empty or it happens to be empty now but in principle it could be non-empty.

(But it's still a fallacy of grey: even if all academic disciplines were, in fact, BS disciplines, some disciplines may still be less BS than others.)

The category being separated is "disciplines", which divides into "BS" and "non-BS". "Academic" disciplines are thus a further subcategory of "BS" disciplines.

Actually, "academic" disciplines would probably be a subcategory of "disciplines" which is largely but not entirely subsumed by "BS" disciplines, but I don't usually demand that level of precision from witticisms.

[For the record, separating a category into two subcategories and proving one of them empty is just another way of proving the original category is identical with the non-empty subcategory. It is, indeed, valid from a technical perspective.]