Luca returns the hug, even though he's confused about what sparked that. Until Luz said something about Ercole and Giulia.

"Oh. I think I mentioned I knew a human that showed me what a phone was, though."

That's...not quite the same thing as saying he knows a 'super cute friend besides Alberto', though. And he certainly hadn't mentioned Ercole.

"I'm guessing you saw the memory through the Winter Mourning, then?"

By this point, he's seen several instances of people seeing memories by touching those things, so it's the most logical conclusion. Either that, or maybe someone else who had told her about it.