Marvin still felt that it was dangerous to use the Book of Nalu.
Even though the Rebirth page was relatively safe, that was when compared to the other pages. He nearly got fucked by the book of Nalu last time with Madeline, almost ending up becoming its slave.
Thus, he remained at a respectful distance from it. The God of Deception wasn't benevolent, and the way he fell was very strange. And Berserk Lord Angola exploding was good enough proof of the Book of Nalu's dangerous nature.
But he didn't have much choice this time.
The fake Hathaway made by Dark Phoenix was a physical body that evolved from her Divine Source. If he wanted to trick Dark Phoenix while taking care of that problem, he could only use the treasure left behind by the God of Deception.
Marvin was the master of that page of the Book of Nalu, and since he had that [Spirit Armband] raising his willpower, his control over the Book of Nalu had increased quite a lot.