Now it should be said of course that one group is actually right

I think this ignores the whole concept of probability.

If one group says tomorrow it will rain, and another group says it will not, of course tomorrow one group will be right and one group will be wrong, but that would be not enough to mark one of those groups irrational today. Even according to best knowledge available, the probabilities of raining and not raining could possibly be 50:50. Then if tomorrow one group is proved right, and another is proved wrong, it would not mean one of them was more rational than the other.

Even if we are not talking about a future event, but about a present or past event, we still have imperfect information, so we are still within the realm of probability. It is still sometimes possible to rationally derive different conclusions.

The problem is that to get perfect opinion about something, one would need not only perfect reasoning, but also perfect information about pretty much everything (or at least a perfect knowledge that those parts of information you don't have are guaranteed to have no influence over the topic you are thinking about). Even if for the sake of discussion we assume that Ayn Rand (or anyone trying to model her) had perfect reasoning, she still could not have perfect information, which is why all her conclusions were necessarily probabilistic. So unless the probability is like over 99%, it is pretty legitimate to disagree rationally.