The common grapefruit diameter is smaller than the diameter of a DVD. People like to reference the size people in the area might be more familiar. When you say grapefruit, I don't think of something the size of a DVD. That would be an extra large grapefruit based on the sizes and pricing of all of the stores that offer grapefruits I have ever seen. The DVD diameter sized citrus fruit I'm familiar would be pomelos, and not all stores carry them.
If you routinely see grapefruit with the same diameter as a DVD that it is a normal measurement for you then, that's great for you, but it is not the normal
The flat part wasn't confusing, was just quibbling about the odd over-explaining of the size (fwiw in the video the person says "baseball" and "softball", and indeed I didn't see any DVD-sized ones).