It won't matter whether or not the answer makes any sense, either. The diameter of 10 cubits is from outer rim to outer rim, the way anyone would measure a circular object. This would be the number needed to calculate the volume of water. There are a couple of problems here. First, the relevant verses do not report measurements being made both around the outside of the bowl and around the inside.
All verses report exterior measurements.
It should be clear from the facts just presented that 8.5 gallons of water can't possibly fit into a cube only 8.83 to 10.11 inches on a side. In fact, a cube 12.52 inches on a side would be required. One can only assume that the writer's math is as poor as his reasoning. Third, Hovind's argument is not original with him, so he could hardly have figured it out in order to resolve any doubts about the accuracy of the Bible. Basically, he accepted somebody else's apologetic and related it without giving any credit.