A while back, when Klaus spoke with Lady Destiny, she told him that although the universe's core had died, not all the laws were corrupted.
Her showing him the three futures was a sign that not all the universal laws had indeed been corrupted. So, before this battle, he knew with certainty that the laws that remained uncorrupted would fight back.
And they did.
They held back the Primordials of the Chaos Race and even many Celestials and Pseudo-Primordials.
Klaus wasn't naive.
He knew, deep down, that the Chaos Race had more Primordials simply because they weren't from this universe.
While corrupted, the laws of this universe understood the danger of Primordials popping up more frequently, so restrictions had been put in place long ago to limit their numbers and activities.
However, places like the Void Realm, Chaos Realm, and even the Forbidden Realm had no such restrictions, so they could easily become Primordials.
