The context of the post is someone trying to get someone else fat. The context of the comment you replied to is talking about how sugar would be a better than fat towards that goal. Sugar is relevant because while you are correct that if you ate an additional equal weight of fats vs sugars, assuming equality of nutrient absorption, the person who ate additional fats would in theory gain weight. The issue here is that this theory does not reflect reality well enough. In reality, we observe patterns like how eating sugar will make you hungrier, causing you to eat more overall, and potentially creating a self-reinforcing cycle of weight gain. This argument is why people are talking about sugar and why it is relevant to the conversation's context.