He's probably talking about the 1lb-Bar2 product (link).
His calculations are not wrong, but it must be considered that he used a ruler in cm to measure the dimensions and the product may have been designed in inches, so so there could be an inaccuracy in the data used. It must also be considered that there are incisions and imperfections in the edges.
In short, the method to find the volume is not very precise. However, this does not mean that the product information may not be correct, as the error indicated in the review is quite big and I don't know if it can be filled with inaccuracies
I found the item on amazon and you can either buy it as a cube or a bar shape. Looks like OP is doing his calculations using the bar and not the cube that's shown.
Amazon reviews are really dumb when you have multiple items in one page because it shows all of them together however if you do pay attention right at the beginning of the review it does say that particular buyer bought the bar form.