Izen had already seen the Infernal King use his flames to destroy one of his Clones. And with everything else that he saw later, it was clear that the flames weren't made using any magic.
That could only mean that they were the real Bloodline abilities of the Infernal King. As much as he didn't want to believe that the Infernal King was a summoned beast, he knew he had to treat him like one. He couldn't leave everything to his Numen which was ineffective against the Infernal King's Flames.
As soon as the Infernal King attacked again, the real Izen took action. This time, he didn't leave everything to luck and cast his strongest Barrier around him and all his clones, one that could especially stop the Infernal Flames.
