إما مركب من جزئين فصاعدا، وهو الجسم
A substance may be composed of two parts or more; this is a body.
On this definition, two or more parts suffice for composition. It does not immediately follow that an observable extension in every direction must be demonstrable. This is precisely the source of the dispute.
The debate is not merely about how bodies look in ordinary experience. It asks what belongs to the technical meaning of jism: composition as such, or the realization of all three dimensions?