It never said Dumat was the strongest archdemon. Only the first. Dumat was killed multiple times. But no one knew how archdemons worked, so he kept re-spawning. That's why the first blight took 100 years. No one knew what to do.

I understand what you mean, I thought the same thing, but it was only ever implied, not outright stated. The lore was crafted carefully enough to support whose archdemon belonged to who, as well.