i think he already knew when they first met in garden. and athy also have some thought about lucas knowing her secret. but yes, this chapter lucas said it clearly in the line.
She doesn't know right now ,but he always did. He might even been the one to bring her to the other dimensions and back to this one.
Remember when athy called Ezekiel hot? Lucas told her "Gross think about your age." He also told her that he is better because he is older. And then he went back to his older state.
Since Lucas and Ezekiel both have a crush an Athy, Lucas tried to one-up Ezekiel by saying he is an adult to. Athy didn't get it, but Lucas was basically telling her that he knows her age.
I never noticed that, me and my dumbass self ╮(￣▽￣)╭
Everyone is talking about Claude and Athy, and not the fact that Lucas knows that Athy traveled through dimensions and time? Ok. I mean there were a few hints to Lucas knowing that Athy had another life but it was just confirmed!!! And we could’ve just assumed he’d seen the soul’s age.