لم تماسه إلا بجزء غير منقسم
It would touch it only by an indivisible part.
The proof assumes that contact across two or more constituents would create an actual extended line or surface at the contact and conflict with ideal roundness. The contact is therefore treated as indivisible.
Yet two questions differ: Is contact represented geometrically by a point? and Is that point an independently subsisting physical atom? The first does not entail the second without a further premise.