Edward realized way back that Rex was the first one to have his view altered.
He was the one to realize that Supernaturals weren't all that bad, there were some that were good and couldn't be compared with the rest. It was shown subtly when he decided to spare Ugrok who was evidently a full-blown Supernatural.
Not to mention, Ugrok is an extremely rare breed of Supernatural.
If he was left unchecked, he could become a big threat to humanity in the near future.
But nevertheless, Rex spared him which was quite shocking to see.
From the moment Edward saw Ugrok, he instantly suspected that Rex had a change of heart.
Now, his assumption holds true as Rex vies to create a Kingdom that accepts all.
Humans or Supernaturals, all are welcomed to his Kingdom.
Going from hating all Supernaturals deeply into accepting them was a big progress—and that progress shows Rex's realization that it was wrong for him to associate all Supernaturals' evil with the sole act of a few Supernaturals.
