If Luke is a Jedi, he must have been trained by a Jedi, in which case it's not strange that he'd know about the Purge. Cal might have preferred to not bring attention to it, though. Even if he's a lot better than he was even just a few months ago, it's still an open wound.

And as kind as Luke seems, they've only just met. So Cal offers a smile, and says:

"No. I'm okay." Time to move on. "Why did they take your lightsaber?"

Because he still has his.