To me it sounded more like a reaction to Bjarne phrasing himself in an unexpected way, much like you'd laugh at a joke. But I regardless do agree that it's inappropriate to laugh at a sincere question. And as an extension to that, it would have been better if Bjarne's first sentence wasn't phrased as a joke.
But I don't think there's any issue with plainly stating that you think the asker is wrong, or with the rest of Bjarne's response. Of course, I'm not a native speaker.
To me it sounded more like a reaction to Bjarne phrasing himself in an unexpected way, much like you'd laugh at a joke. But I regardless do agree that it's inappropriate to laugh at a sincere question. And as an extension to that, it would have been better if Bjarne's first sentence wasn't phrased as a joke.
But I don't think there's any issue with plainly stating that you think the asker is wrong, or with the rest of Bjarne's response. Of course, I'm not a native speaker.