How can the dude be a century old monster and not know how to lie any better than that? He didn't even need to lie in the first place, which is always better than trying to come up with blatant lies extempore. Ru Meng ought to know by now that Yang Kai was detained by Bei Li in the ice prison. He could have told her that on the lowest level, he met the lingering soul of a person who refused to tell her name, but she wanted revenge on Bei Li and gave him a single use weapon, which supposedly would hurt Bei Li even if it couldn't kill a demon saint. He used it, it worked as promised to pause the fight for a moment. Then Ru Meng already arrived and saw the rest. It would be a true story, apart from the parts edited out.
But I guess the author wants to limit Yang Kai's power by giving him occasional weaknesses.