The Background to line 24 finishes with "Rebbi Yochanan limits it to objects that are sold only by count, and not by estimate". What do we mean by "estimate"? Isn't everything supposed to be exact and wouldn't weighing be considered as "exact" if fair weights and measures were used?
Weighing something does not give significance and importance to the commodity being weighed, so that it will not become Batel b'Rov, for there is no focus on each individual item. It yields an exact weight, which can consist of any number of items. Secondly, usually an extra bit was thrown in "for good measure" (called Hachra'ah, Bava Basra 88) when an item was weighed or measured.
On the other hand, when the commodity is counted one by one, it shows that each item if valuable. Since the prohibited item that was mixed in is of substantial importance in its own right, it will not become Batel b'Rov.