The outside observer cannot be sure. And that is the point. One could never have been sure about what path the Opportunists would take. It is as if Dante the Writer is telling us this place warrants little attention, and he actually spends few lines on it. One should look elsewhere.
Feedback is good, too.
In fact, the punishment of the Vestibule is indeed one of the most hideous in all of Hell, but it serves some wonderful purposes as well. If it were more profitable to choose good, they did so. If the coins flowed more readily from the side of evil, the opportunists would choose that. Even the souls in Hell have the dignity to know they are there because they chose it. I suppose it is so that those who enter the gate, on a path to the lower levels, can see what awaits those who had not made a commitment even to evil.