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.

Of course! Thanks :)

He said a cube of 1.8m, not 1.8m3 - he means 1.8m x 1.8m x 1.8m, making it easier to visualise :)