I think the difference here is that NPO decimated dozens of alliances through crippling wars and sanctions and viceroyships over the course of three years. The NPO and it's allies constantly stomped on smaller alliances and would just pat each other on the back over it. To this day many of them have no remorse over anything they did.
Athens fought a couple of cycles of wars against one alliance and then offered peace. Their allies were like "Hey, you're our friends and all, but that wasn't cool." And Athens was like "Yeah I guess that was kinda lame on our part." And then started working to make amends.
Now, considering both of these cases, why do you think Athens deserves some sort of severe punishment? Haven't they done the right thing in admitting the mistake, taking responsibility, and working to make amends?
To say that Athens deserves to be punished harshly leads me to believe you're not interested in justice so much as you're interested in spilling more blood.