Nevertheless, I think that Tastidian believed that Noctus could win. Otherwise, he would have made further precautions to be assured that the Main and David couldn't leave the camp after the
test with Noctus. Two worthy slaves are better than one

Tastidian going to the Psy Academy surely required huge amounts of obfuscation and a well placed shadowport. And he probably didn't have the necessary power to capture both David and the Main in one go... or he was so impressed with the powers of the Main that he decided that the Main should be rewarded somehow, by granting the Main the possibility of letting David be free.
Anyway, I agree that Noctus proved himself worthy of being revived by Nelis.