Meng Fuyao was waiting.
She was waiting for the ash to fade away completely.
That bastard could not possibly be so legendary to be able to exactly replicate someone else. As such, he would always use smokescreen whenever he appeared. The first time was in the dark cabin, while the second time was when he walked so quickly that the pile of fire surged and blurred their vision. This time, he made use of the moment that she closed her eyes to destroy the wooden railings. When ash was all over the place, he capitalized on the opportunity to appear at the same moment as Zhangsun Wuji did.
The ash would eventually fade, and there would definitely be a flaw in his trickery.
