If 2 Maccabees teaches praying for the dead after mortal sin, it overturns Catholic theology on mortal sin and purgatory

Mike presses the point: if second Maccabees is about purgatory, you could pray for someone who committed a mortal sin and died without absolution. This would nullify the entire Catholic distinction between mortal and venial sin — there would be no sin serious enough to exclude someone from being helped by the prayers of the living.