"No, I can't know." Jayce allows that denial. It could be true. She could be mistaken, she didn't say she saw his body. She could be from a different timeline. These are all facts, but it's not what he feels. Jinx attacking the Council after Silco's death, completely reasonable. What she's already managed to make out of the gemstones could absolutely lead to a weapon they can't fight. He made a hammer no one could fight. With Jinx's mind? She could blow up all of Piltover.

"But does it matter? Let's say it's a 50-50 shot it's true. Are those the odds I want to play with my life? No." Jayce is not talking about them intentionally going back because he never had any plans for that, although he knew Viktor felt differently. Maybe that is why he is so calm now. If the Singularity sends him back, it's out of his hands, but he feels better, in a way. He's already embraced this life as the one he wants, and now he doesn't have to wonder, if he doesn't want to.

That is why he is showing more comfort to Viktor than him. Jayce has already been here. The fact he's probably dead is just a literal nail in the coffin. "I told Vi this is a place for second chances. I have to go with the assumption this is it for me, Viktor." Jayce has been saying that about Viktor this entire time, he is going to die back home, there is no hope. Now it's done for both of them.