He can't help himself. He laughs. Hearing Gon awkwardly call it "the dream" is just a bit too much not too. Not to mention, he hears what's implied there.

"My apologies, my friend. I've always been something of a writer, interested in language, I don't always think about what idioms aren't so easily caught. It's probably because many of them are really old when they were said first, making more sense in the time period or being a way to sort of walk around things that couldn't be said otherwise."