Olam Haba is later on. We don’t know what is going on right now with the fellow. But, even if he is somewhere low down, saying a Kaddish for his merit can alleviate his situation somewhat. And as Sam pointed out, Chazal make sweeping statements because these are the rules, but there are always other things taken into consideration when it comes to practice. We must follow Halacha to the T because we don’t know and work with Taama d’Kra. In heaven there are sliding scales.
We find this even by the Sota, where the whole purpose is to be a clear proof, and yet we find that certain Zechusim can protect the woman even when she is guilty.