No, the grandparent was right. He's talking about how big a cube could contain 6000L, but you're calculating the volume of 6000L. Simple way to visualize this:

No, the grandparent was right. He's talking about how big a cube could contain 6000L, but you're calculating the volume of 6000L. Simple way to visualize this: Think about the difference between a 4-foot square and 4 square feet. The former is a much bigger area.