The Gmara concludes that the 24 people of R' Yochanan are sitting outside of the circle so that the circumference of the circle that goes around them has a diameter of eight, not six. If, however, you put 24 people around the inside of a circle with a diameter of eight, those same 24 people, who are assumed to be an ama by an ama in all directions, will be up against the inner circle which only has a diameter of six. Since the gmara already concluded that 24 people won't fit around a circle with a diameter of six, what did the gmara gain by answering that they are outside the circle? They are still up against it!
Avrohom Dubin, Brooklyn, NY
The Gemara's answer could indeed be phrased in many ways, whether it is outside (Rashi), inside, or that the men are the walls of the circle (Rabeinu Chananel). The Gemara is not focusing on this aspect, but rather the fact that there are six Amos in between the men. This gives us a circle of eighteen, and the Gemara's answer.
All the best,
Yaakov Montrose